Optical amplifiers and light source

ABSTRACT

Single- or few-moded waveguiding cladding-pumped lasers, superfluorescent sources, and amplifiers, as well as lasers, including those for high-energy pulses, are disclosed, in which the interaction between the waveguided light and a gain medium is substantially reduced. This leads to decreased losses of guided desired light as well as to decreased losses through emission of undesired light, compared to devices of the prior art. Furthermore, cross-talk and inter-symbol interference in semiconductor amplifiers can be reduced. We also disclose devices with a predetermined saturation power. As a preferred embodiment of the invention, we disclose a single (transverse) mode optical fiber laser or amplifier in which the active medium (providing gain or saturable absorption) is shaped as a ring, situated in a region of the fiber&#39; cross-section where the intensity of the signal light is substantially reduced compared to its peak value. The fiber can be cladding-pumped.

RELATED CASES

This application is a Continuation-in-part of co-pending PCT PatentApplication No. PCT/GB97/03353, filed Dec. 4, 1997, which claimspriority to Great Britain patent application Serial No. GB962531.7,filed Dec. 4, 1996, now abandoned.

FIELD OF THE INVENTION

The invention relates to optical amplifiers and light sources. By way ofexample, though not exclusively, the invention relates to single- orfew-moded waveguiding lasers, superfluorescent sources, opticalamplifiers, high pulse-energy devices, energy-storage devices,cladding-pumped devices, semiconductor signal amplifiers, andwaveguiding saturable absorbers.

BACKGROUND OF THE INVENTION

The tightly confined modal fields of single- or few-moded waveguidinglasers, superfluorescent sources, and amplifiers lead to a very stronginteraction between any waveguided light and the active medium in thewaveguiding core. Therefore, a comparatively small amount of gain mediumis sufficient for providing the gain in these devices. Specifically, thegain for a given stored energy, as well as for a given absorbed pumppower, is high. This is often beneficial, since it means that the pumppower requirements for a given desired laser output power or amplifiergain can be low.

However, for several devices, this efficient interaction between modeand gain medium can be detrimental. The following example refers tocertain types of amplifiers and lasers, but of course the skilled manwill realise that the same or similar problems can occur in, forexample, superfluorescent sources.

In a laser or amplifier, the achievable single-pass gain is limited to,say, 50 dB. The reason is that at this gain, a significant fraction ofthe pump power is converted to amplified spontaneous emission (ASE). A10 dB higher gain results in approximately 10 dB more ASE, so at thesegains, the extra pump power required to increase the gain further willbe prohibitively high. Since the ASE limits the gain of the device, italso limits the energy stored in the gain media. This in turn obviouslylimits the amount of energy that a pulse can extract from the device.Consequently, the pulse energy that can be obtained from waveguidinglasers and amplifiers is limited. Instead, bulk (i.e., not waveguiding)lasers and amplifiers for which the extractable energy for a given gaincan be several orders of magnitude lower are often employed to providemuch higher pulse energies. However, the robustness and stability ofbulk lasers is often inferior to waveguiding ones.

Moreover, the gain limit can also be problematic for lasers andamplifiers irrespective of whether the stored energy is a major concern,if the high gain appears at another wavelength than the desired one. Thereason is that ASE (or lasing) at the gain peak will suppress the gainachievable at the desired wavelength, possibly to a value below what isrequired for a good amplifier or laser. This applies to all types ofamplifiers and lasers.

Furthermore, in optically pumped lasers and amplifiers, a suitableinteraction between the gain medium and the amplified or generatedsignal beam is not enough; also the interaction between the pump beamand the gain medium must be appropriate. However, in some types oflasers and amplifiers (typically cladding-pumped ones), the interactionwith the pump beam is significantly smaller than the interaction withthe signal beam. Then, for a device that efficiently absorbs the pump,the interaction with the signal beam will be much stronger than what isrequired. Unfortunately, this excess interaction is often accompanied byexcess losses for the signal beam, since:

1. The scattering loss of an active medium is normally higher than itcan be for a passive medium. For instance, rare-earth-doped fibers havescattering losses of, e.g., several orders of magnitude higher thanstandard, passive, single-mode fibers.

2. A fraction of the active medium often has inferior properties. Forinstance, in Er-doped fibers, pairs of Er³⁺-ions can form. These resultin an unbleachable loss. The strong interaction then leads to a highloss.

3. The active medium in its amplifying state can also absorb light(so-called excited-state absorption, ESA). Again, a stronger interactionleads to more power lost through ESA.

Moreover, a bleachable medium (e.g., an unpumped gain medium with aground-state absorption) can be used as a saturable absorber. Anefficient interaction leads to a low saturation power. A reducedinteraction leads to a higher saturation power, which can be moresuitable for some applications, especially if the interaction, and hencethe saturation power, can be controlled.

Clearly, although often beneficial, the tight confinement of the guidedlight is a problem for some devices.

SUMMARY OF THE INVENTION

An aim of the present invention is to improve the interaction betweenlight guided along a waveguide and rare-earth dopants within an activemedium.

Accordingly in one non-limiting embodiment of the present invention,there is provided apparatus comprising a waveguide and an amplifyingregion wherein the waveguide comprises a core and a cladding and theamplifying region comprises rare-earth dopants and wherein theamplifying region comprises a ring around the core of the waveguide.

Various aspects of the invention are defined in the appended claims, andin passages throughout the present application.

According to a first embodiment of the present invention, there isprovided an amplifying optical device comprising a first waveguidingstructure comprising a first core and cladding and configured to guideoptical radiation, at least one pump source configured to supply opticalpump power, an amplifying region situated in the cladding; and whereinthe pump source is optically coupled to the amplifying region; andwherein in use the optical radiation guided in the first waveguidingstructure overlaps the amplifying region.

The invention also provides a method of pumping at least one opticalfiber amplifier with a fiber laser, the method comprising providing afirst waveguiding structure fabricated from at least one glass systemand comprising a first core and cladding; providing a second waveguidingstructure comprising a second core at least partly formed by thecladding and an amplifying region comprising Ytterbium; providing asource of optical pump power in optical communication with the secondwaveguiding structure and having a wavelength in the band from about 870nm to about 950 nm; providing an optical feedback device; guidingoptical radiation using the first waveguiding structure; guiding theoptical pump power using the second waveguiding structure such that theamplifying region interacts with the optical radiation guided in thefirst waveguiding structure and the optical pump power guided in thesecond waveguiding structure to amplify the optical radiation guided bythe first waveguiding structure; using the optical feedback device toensure that a plurality of times a portion of the optical radiationguided by the first waveguiding structure is amplified more than once bythe amplifying region; providing an amplifying region characterized by adopant concentration, a disposition and a length, and wherein the dopantconcentration, the disposition and the length of the amplifying regionare arranged such that the fiber laser emits optical radiation at anemission wavelength in the region of about 970 nm to about 990 nm; andcoupling the optical radiation at the emission wavelength in the regionof about 970 nm to 990 nm into the at least one optical amplifier.

A second method provided by the invention is a method of amplifyingoptical pulses to energies exceeding the intrinsic saturation energy ofan amplifying optical device, comprising: providing a first waveguidingstructure comprising a first core and cladding; providing a source ofoptical pump power; providing a second waveguiding structure comprisinga second core at least partly formed by at least part of the cladding,and an amplifying region; guiding optical radiation using the firstwaveguiding structure; and guiding the optical pump power using thesecond waveguiding structure such that the amplifying region interactswith the optical radiation guided in the first waveguiding structure andthe optical pump power guided in the second waveguiding structure.

A third method provided by the invention is the method of using awaveguiding saturating absorber comprising: providing a waveguidingstructure having a core and a cladding; guiding optical radiation in thewaveguiding structure; providing an absorbing region situated within thecladding and disposed such that it provides an absorption of the opticalradiation guided in the core such that in use at least 10% of theabsorption is bleached by the optical radiation guided by the core in atleast a part of the waveguiding saturating absorber at least part of thetime.

According to a second embodiment of the present invention, there isprovided an amplifying optical device comprising: a first waveguidingstructure configured to guide optical radiation which can propagate in afundamental mode; a pump source configured to supply optical pump power;and a second waveguiding structure configured to guide the optical pumppower, wherein the pump source is optically coupled to the secondwaveguiding structure; and wherein in use the optical radiation ischaracterized by an optical power distribution of the fundamental modehaving a contour of equal intensity perpendicular to the locallongitudinal axis of the first waveguiding structure the contourenclosing about 75% of the optical power of the fundamental mode; andwherein the second waveguiding structure contains an amplifying regionsituated to interact with the optical pump power guided in the secondwaveguiding structure when the amplifying optical device is in use; andwherein the amplifying region is situated to lie outside the contour ofequal intensity; and wherein during use at least 0.1% of the opticalradiation guided by the first waveguiding structure overlaps theamplifying region.

Embodiments of the invention provide devices that are considerablyimproved by a predetermined reduction of the interaction between asignal light beam and an active medium (per unit volume) compared toprior-art designs, without necessarily changing the properties of thegain medium or reducing the confinement of the signal light (although areduced confinement can also be beneficial for the disclosed devices).The active medium serves to amplify or generate the signal light beam,or, if unpumped, can act as a saturable absorber.

The reduction in interaction is achieved by placing the bulk of theactive medium in regions where the intensity of the signal beam issubstantially smaller than its peak intensity, in a cross-section of thewaveguiding device perpendicular to the direction of propagation of thesignal beam. This can provide advantages for the following devices:

1. Lasers (e.g., Q-switched and gain-switched ones) and amplifiers inwhich it is desirable to store large energies. In these devices (as wellas for so-called energy-storage devices in general), the reducedinteraction leads to a larger stored energy before practical upperlimits on the gain is reached.

2. Optical amplifiers (typically semiconductor ones) for which even theenergy of a single signal bit can be comparable to the stored energy. Inthose, already the amplification of a bit extracts enough energy toreduce the gain. This leads to four-wave mixing, cross-talk, andinter-symbol interference. This can be reduced with the higher storedenergy that, for a given gain, accompanies the reduced interaction.

3. Amplifiers and lasers in which an efficient pump absorptionnecessitates large amounts of gain media, which in prior-art devicesleads to excessive small-signal absorption, background absorption, orexcited static absorption at the operating wavelength, or excessive gainat another wavelength. A reduced interaction then leads to reducedlosses. Moreover, a reduced interaction can reduce the gain at theundesired wavelength relative to that at the desired one, and therebythe problems associated with a too high gain at the wrong wavelength.This applies to lasers in which there is a significant unpumped loss(typically, reabsorption loss or out-coupling loss). These points areespecially relevant for cladding-pumped devices. For example, to ensuresufficient pump absorption, the fiber may need to be so long that one orboth of those problems arise.

4. Saturable absorbers, in which the saturation power is otherwise toosmall.

Embodiments of the invention can overcome or alleviate some of theproblems described above and can at least partially achieve one or moreof the following:

1. To reduce the susceptibility to so-called quenching and backgroundlosses, in particular for cladding-pumped devices.

2. To obtain efficient emission at wavelengths otherwise inaccessiblefor devices where there is a significant unpumped loss, in particularfor cladding-pumped devices.

3. To improve the energy storage capabilities, for energy-storagedevices.

4. To reduce signal cross-talk and inter-symbol interference for signalamplifiers.

5. To allow for a larger, predetermined, saturation power.

Embodiments of the invention can provide the following devices andembodiments, and the use of the following amplifying and/or absorbingwaveguiding structures in such devices:

1. An amplifying optical fiber in which the active medium is placedpartly or wholly outside the waveguiding core, e.g., in a ring aroundthe core. The gain medium can also reside inside the core in regionswhere the normalized modal intensity of the signal beam is small. Thefiber can be made of a glass, partly doped with Pr³⁺, Tm³⁺, Sm³⁺, Ho³⁺,Nd³⁺, Er³⁺, or Yb³⁺, or a combination thereof, and it can becladding-pumped.

2. A cladding-pumped amplifier or laser in which the difference betweenthe overlaps of the pump and signal beams with gain medium issubstantially reduced compared to prior-art designs.

3. A ring-doped optical fiber for high-energy pulse amplification orgeneration or other energy storage applications. The fiber can forinstance be made of a glass, partly doped with Pr³⁺, Tm³⁺, Sm³⁺, Nd³⁺,Nd³⁺, Er³⁺, or Yb³⁺, or a combination thereof, and cladding-pumped.Moreover, the device can incorporate a longitudinally distributedsaturable absorber to suppress the build-up of ASE. In one embodiment,the gain medium is a Yb³⁺-sensitized Er³⁺-doped glass, and the saturableabsorber is an Er³⁺-doped glass, and they are located so that the signalintensity is higher in the saturable absorber than in the gain medium.

4. A Q-switched or gain-switched fiber laser based on an amplifyingfiber with a relatively higher saturation energy combined with asaturable absorber fiber having a relatively lower saturation energy.The difference in saturation energy stems, at least to a significantpart, from differences in the geometry of the fibers. The active mediain the different fibers can be the same or different, and can forinstance be a glass doped with a rare earth, e.g., Pr³⁺, Tm³⁺, Sm³⁺,Ho³⁺, Nd³⁺, Er³⁺, or Yb³+, or a combination thereof.

5. A ring-doped, cladding-pumped ytterbium-doped fiber for amplificationor generation of light in the range 950 nm to 1050 nm.

6. A ring-doped, cladding-pumped neodymium-doped fiber for amplificationor generation of light in the range 850 nm to 950 nm.

7. A ring-doped, cladding-pumped erbium-doped fiber for amplification orgeneration of light in the range 1450 nm to 1600 nm.

8. An amplifying planar waveguide structure in which the active mediumis placed partly or wholly outside the waveguiding core, thusinteracting with the signal beam only where the normalized intensity ofthe modal field is small. The waveguide can be cladding-pumped.Moreover, the design can be specifically adapted to correspond to any ofthe fiber devices listed above.

9. A semiconductor amplifier for signal amplification, in which the gainregion is placed partly or wholly outside the waveguiding core, thusinteracting with the signal beams only where their normalized modalintensities are small. Thereby, the saturation energy of the device willbe increased, which subsequently reduces the inter-symbol interferenceand inter-wavelength cross-talk.

10. A waveguiding structure with a saturable absorption, in which theabsorbing medium is placed partly or wholly outside the waveguidingcore, thus interacting with the signal beam only where its normalizedmodal intensity is small.

Evanescent-field devices, including ring-doped fiber devices have notbeen considered for devices of the type proposed here, nor has anydevice been proposed or demonstrated based on ring-doping (or evanescentfield interaction) that provide significant benefits of the typeconsidered here, compared to traditional devices in which thegain-medium resides in the core in places where the interaction with thesignal beam is large. Specific differences between embodiments of theinvention and a prior art device are as follows:

1. It has not been one of the specific devices considered here.

2. It has not used a single-moded or few-moded waveguiding core.

3. It has not been a device in which the energy extraction results incross-talk or inter-symbol interference.

4. The control of the emission wavelength that we propose has not beenobtained.

5. The device has not substantially reduced the effect of losses at thesignal wavelength.

6. It has not been a cladding-pumped device.

7. It has not been a device for high-energy pulses.

8. It has not been an optical fiber doped with erbium or anotherrare-earth for high-energy pulses.

9. The output of the device could not be launched into a standardsingle-mode fiber through splicing or butt-coupling, nor has the deviceallowed for an easy launch of signal light.

10. The output beam has not been tightly confined.

11. It has been a device doped in regions of the core where the modalintensity is large.

12. It has been a device doped in a large area around the core (e.g.,homogeneously in the cladding), hence rendering it inefficient forcladding-pumping.

13. It has not been a fiber structure, or at least not an all-fiberstructure.

14. It has not been a solid-state device.

15. The interaction length has been limited to a few centimeters.

16. It has not been a high-gain device.

17. It has not been a device pumped by an optical beam guided along theamplifying medium.

18. It has not been possible to manufacture the device with standardmanufacturing techniques for rare-earth doped fibers like MCVD andsolution doping.

19. The purpose of the design has not been to obtain a smallerinteraction between the gain medium and the signal light than wouldotherwise be possible, nor have any substantial benefits of asubstantially smaller interaction been proposed, discussed, ordemonstrated.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference tothe accompanying drawings, throughout which like parts are referenced toby like references, and in which:

FIG. 1 illustrates a Ring-doped optical fiber;

FIG. 2 illustrates the dependencies of the refractive index, the gainmedium, and the modal field across a transverse cross-section throughthe center of the fiber in FIG. 1;

FIG. 3 illustrates a planar waveguide structure amplifying theevanescent field of a signal beam;

FIG. 4 illustrates a double-clad ring-doped optical fiber;

FIGS. 5a and 5 b illustrate examples of the proposed devices;

FIG. 6 illustrates the extractable energy and small-signal gain at 1550nm for a ring-doped erbium-doped fiber (EDF) pumped by 0.1 W, 0.2 W, and0.5 W at 1480 nm in the core;

FIG. 7 illustrates the extractable energy and small-signal gain at 1550nm for a ring-doped erbium-doped fiber (EDF) pumped by 0.1 W, 0.2 W, and0.5 W at 980 nm in the core;

FIG. 8 illustrates the normalized modal intensity Ψ vs. ring positionfor the ring-doped EDFs of FIGS. 6, 7, and 10;

FIG. 9 illustrates the extractable energy (“pulse energy above cw”) vs.launched pump power for a core-pumped fiber amplifier with an Yb³⁺-dopedring;

FIG. 10 illustrates the extractable energy and small-signal gain at 1550nm for a ring-doped EDF cladding-pumped by 1 W and 5 W at 980 nm;

FIG. 11 illustrates a view of a fiber having a saturable absorber in thecentral part of the core and a ring-shaped gain medium around theabsorber;

FIG. 12 illustrates a semiconductor amplifier for signal amplification;

FIG. 13a to 13 c illustrates devices in which unwanted, higher-ordermodes are suppressed by the inclusion of an absorber;

FIG. 14 shows an amplifying optical device;

FIG. 15 shows an amplifying optical device containing a secondwaveguiding structure;

FIG. 16 shows a preferred embodiment of an amplifying optical device inwhich there is a significant saturable small signal absorption;

FIG. 17 shows a schematic of a high-power optical amplifier;

FIG. 18 shows a master oscillator power amplifier MOPA;

FIG. 19 shows a fiber laser;

FIG. 20 shows a fiber laser which includes a reflector;

FIG. 21 shows a preferred embodiment of a fiber laser containingYtterbium as a dopant;

FIG. 22 shows a Q-switched laser;

FIG. 23 shows a fiber laser being used to pump three optical fiberamplifiers;

FIG. 24 shows a waveguiding saturating absorber;

FIG. 25 shows an amplifying optical device in which an amplifying regionis situated outside the first core;

FIG. 26 shows a passive Q-switched laser;

FIG. 27 shows a passive Q-switched laser which includes a pumpreflector;

FIG. 28 shows an optical fiber having a ring-doped amplifying regionlocated in the cladding;

FIG. 29 shows a wavelength-tracking filter; and

FIG. 30 shows a single-frequency laser.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 depicts a ring-doped optical fiber. A transparent cladding (10)(typical radius 50 μm-250 μm) surrounds a transparent waveguiding core(30) of a higher refractive index, with a diameter of typically a few toten μm (micrometers). The core is surrounded by a gain medium (20),which can amplify a signal beam, guided by the core. The gain medium(20) can be pumped by an optical pump beam, which can amplify a signalbeam, guided by the core.

FIG. 2 illustrates the normalized modal intensity distribution Ψ, therefractive index profile with the core (30), and the dopant profile(20), in a transverse cross-section through the center of the fiber. Foran optical fiber in glass, the cladding refractive index is typicallyaround 1.5, and the numerical aperture is typically around 0.1-0.3.

FIG. 3 shows a waveguiding amplifier or laser. As for the fiber, atransparent cladding (110) surrounds a transparent waveguiding core(130) of a higher refractive index. A gain medium (120) is situated nearthe core.

FIG. 4 is similar to FIG. 1, except that the inner cladding (10) is nowsurrounded by an outer cladding (210), of a lower refractive index.Thus, the inner cladding can guide light, and serves to guide a pumpbeam launched into the inner cladding. The signal beam is guided by thecore (30).

FIGS. 5a and 5 b illustrate examples of an erbium-doped fiber amplifierand a fiber laser respectively. For the amplifier of FIG. 5a, a signalbeam in an optical fiber is launched into a wavelength-selective coupler(310). Also an optical pump beam from the pig-tailed pump source (320)is launched into the coupler, which combines the pump and signal beamsand launches them both into an erbium-doped fiber (330). In the fiber,the erbium-ions serve to transfer energy from the pump beam to thesignal beam, which is thereby amplified. The amplified signal is then,for example, launched into another fiber for further transmission.

For the laser of FIG. 5b, a beam from an optical pump source (370) iscoupled via a lens (350) into a fiber (340) doped with a gain medium.The ends of the fiber provide some means (360) for reflecting a signalbeam, possibly with wavelength discrimination, thus providing feedbackfor the laser. The reflector at the pump input end transmits the pumpand reflects the signal, while the out-coupling reflector in the otherend transmits a significant fraction of the signal beam. Othercomponents are also often used in the devices of FIGS. 5a and 5 b, e.g.,an isolator for the amplifier; however those have been omitted forclarity.

Although it is clear that the ideas and concepts disclosed below applyto many different geometries, the discussion below will for concisenessbe focused on ring-doped fibers. Moreover, it will be assumed that thestructures are longitudinally uniform, although this is not necessarilyso.

Other waveguiding geometries can also be used. For example, the core canbe of a more complicated shape than the traditional ones illustrated inthe drawings. The invention also extends to cores that fulfill the sameor a similar function as traditional ones do, and allow for an activemedium to be incorporated in a region where the normalized modalintensity is small.

Moreover, while the embodiments primarily deal with devices pumped by anoptical beam propagating along the core, other pumping schemes are alsopossible, like flash-lamp pumping and side-pumping with diode bars,electrical pumping, chemical pumping, and more.

While advantages are described mainly in terms of localizing the activemedium in regions where the normalized modal field is small, the activemedium can also extend to regions where it is large.

Principle

The disclosed devices provide advantages compared to prior-art,core-doped, devices by suppressing gain and thus radiation losses atundesired wavelengths and/or by reducing the propagation losses in thedevice. Below follows a description of how these advantages can beobtained. We restrict the discussion to homogeneously broadened gainmedia; substantial benefits can be realized also in inhomogeneouslybroadened devices. The description focuses on cladding-pumped devices.

It is known that with a gain medium for which the shape of the gainspectrum depends on the population inversion, the emission wavelength ofa fiber can be modified by changing the strength of the interactionbetween a signal beam and the gain medium. For instance, the fiberlength can be changed. This also changes the absorption of the pump.However, we will demonstrate below that in cladding-pumped devices, thesame control can be obtained through ring-doping, while separatelycontrolling the absorption of the pump. In particular, the pumpabsorption can be kept sufficiently large, as will be further describedin the following.

The following relation can be used for evaluating the gain G in awaveguiding device with a homogeneously broadened gain medium [see C. R.Giles and E. Desurvire, “Modeling erbium-doped fiber amplifiers”, J.lightwave Technol. 9, 271-83, (1991)]:

n where N₀ is the concentration of amplifying centra, n₂ is the degreeof excitation, Ψ is the normalized mode intensity, σ^(a) and σ^(e) arethe absorption and emission cross-section of the active centra,respectively, and L is the length of the gain region. Equation 1 can bewritten in a simplified form:

G=(10/ln 10)N ₀ A _(doped)Ψ_(doped) [n ₂σ^(e)−(1−n ₂)σ^(a) ]L[dB]  (2)

where N₀, n₂, and Ψ_(doped) have been appropriately averaged over thedoped area A_(doped). (In the literature, the product A_(doped)Ψ_(doped) is often replaced by the so-called overlap Γ.)

There are two assumptions in Eqs. 1 and 2, namely, that the gain ishomogeneously broadened and that only two levels in the gain medium aresignificantly populated. However, even for devices that do not meetthese assumptions, the problems that we address exist and can generallybe countered by designing devices according to our present invention. Inthe notation, there is also the implicit, unimportant, assumption thatthe gain stems from a number of active centra, each of which has beenascribed cross-sections for stimulated emission and absorption. Othertypes of gain media also exist, and the results will be valid also forthem. To proceed, we will also assume that the degree of inversion iswavelength-independent. This is normally true to a good approximation.If not, this results in a slight inhomogeneity in the gain spectrum. Forsimplicity, we have also assumed that other losses are small compared toeither the gain G or the bleachable absorption(10/ln10)N₀A_(doped)Ψ_(doped)σ^(a)L. Again, this is a non-restrictiveassumption, and the equations can be easily modified to include anyother loss. For instance, a filter can be used for controlling the gainspectrum and laser output wavelength, both in prior-art devices and thedevices disclosed here.

It follows from Eq. 2 that the gains G₁, G₂, and G₃ at three differentwavelengths λ₁, λ₂, and λ₃ are related to each other in the followingway:

G ₃ =G ₂(Ψ_(3, doped)/Ψ_(2, doped)) (σ₃ ^(e)/σ₁ ^(e)−σ₃ ^(a)/σ₁^(a))/(σ₂ ^(e)/σ₁ ^(e)−σ₂ ^(a)/σ₁ ^(a) )

+G₁(Ψ_(3, doped)Ψ_(1, doped))(σ₃ ^(e)/σ₂ ^(e)−σ₃ ^(a)/σ₂ ^(a))/(σ₁^(e)/σ₂ ^(e)−σ₁ ^(a)/σ₂ ^(a))[dB]  (3)

Equation 3 makes the important point that for given cross-sections, theonly parameters that affect this relation are the normalized modeintensities, averaged over the doped region. Let now λ₁ be the pumpwavelength. The pump is then absorbed by an amount α_(p)^(operating)≡−G₁ in the operating state of the device. In order tooperate efficiently, α_(p) ^(operating) needs to be sufficiently large,say, at least 5 dB. Also, we assume that we require a certain gain G₂ ata wavelength λ₂. α_(p) ^(operating) and G₂ are then parameters alreadyspecified. This also implies a certain gain G₃ at other wavelengths λ₃,but if this gain is too large, prohibitive amounts of power will be lostto ASE. Insofar as the cross-sections cannot be significantly modified,this can only be remedied by designing the device for appropriate valuesof the normalized modal intensities. The description of such designs isa central part of the present invention.

To simplify the further description, we now assume that the pump doesnot stimulate any emission; hence, σ₁ ^(e)=0. Equation 3 then becomes

G ₃ =G ₂(σ₃ ^(e)Ψ_(3, doped)/σ₂ ^(e)Ψ_(2, doped))+α_(p) ^(operating)(σ₃^(e)Ψ_(3, doped)/σ_(p) ^(a)Ψ_(p, doped))[σ₂ ^(a)/σ₂ ^(e))−(σ₃ ^(a)/σ₃^(e))][dB]  (4)

The value of the first term depends on the relative sizes ofΨ_(2, doped) and Ψ_(3, doped) at λ₂ and λ₃. In a fiber, the spot-sizesat λ₂ and λ₃ can differ. Then, ring-doping implies that the gain at thewavelength with the larger spot-size gets relatively larger than at theother wavelength, compared to a homogeneously doped core. Depending onhow close the wavelengths are to each other, this is often not asignificant effect.

In contrast, in cladding-pumped devices, the second term in Eq. 4 can toa significant extent be controlled by designing the device for anappropriate value of (Ψ_(3, doped)/Ψ_(p, doped)). Normally, it is verydifferent in a cladding-pumped device and in a core-pumped device. Inthe core, the normalized pump intensity Ψ_(p) is approximately equal tothe inverse of the pumped area for both core-pumped and cladding-pumpeddevices, so the same is true for Ψ_(p, doped) in a core-doped device. Itfollows that in a core-doped device, Ψ_(p, doped) will be much larger ina core-pumped device than in a cladding-pumped device. Thus, theeffective area ratio r_(effective)≡(Ψ_(3, doped)/Ψ_(p, doped)) will bemuch larger. (We will also use “effective area ratio” for the ratioΨ_(2, doped)/Ψ_(p, doped).) Consequently, a core design which issuitable for the core-pumped device may be inappropriate for acladding-pumped device because the effective area ratio becomes toolarge. In prior-art cladding-pumped devices, r_(effective) is large,typically around 100. Then, the second term in Eq. 4 is potentiallylarge for some undesired wavelength λ₃, which makes it difficult toabsorb the pump without getting a high gain at the undesired wavelength.Therefore, laser systems with significant reabsorption that work well ina core-doped, core-pumped, geometry will not be efficient core-doped,cladding-pumped lasers. (In a device doped in the core, r_(effective) isapproximately equal to the area ratio r≡A_(pumped)/A_(doped), whereA_(pumped) is the pumped area and A_(doped) is the doped area. Hence,for a cladding-pumped device homogeneously doped throughout the core,r=A_(cladding)/A_(core).)

Consider instead a ring-doped, cladding-pumped device. Since Ψ_(p) isapproximately constant over the inner cladding, Ψ_(p, doped) will notchange much with the transverse disposition of the gain medium. However,since the light at λ₃ is confined to the core, Ψ_(3, doped) decreasesrapidly if the amplifying region is moved away from the core. Thisobviously reduces the interaction between the gain medium and the signalbeam. Hence, the devices disclosed here allows r_(effective) to besubstantially reduced, e.g., to values in the range 1-10, whereby thegain at unwanted wavelengths can be suppressed compared to the gain at adesired wavelength.

First, we treat the case where the scattering (or absorption) loss ofthe gain region is larger than that of a transparent, passive region.For simplicity, we assume that there is no scattering loss outside thegain region. Starting from Eq. 2, we can then derive the followingexpression between the scattering loss and the gain G₁ and G₂ at twodifferent wavelengths:

α₂ ^(scatter)=σ₂ ^(scatter) [G ₂(σ₁ ^(a)+σ₁^(e))−Ψ_(2, doped)/Ψ_(1, doped))G ₁(σ₂ ^(a)+σ₂ ^(e))]/(σ₁ ^(a)σ₂ ^(e)−σ₁^(e)σ₂ ^(a))[dB]  (5)

In Eq. 5, we have arbitrarily made the non-restrictive assumption thateach active center scatters with a cross-section σ₂ ^(scatter). Also, wehave for simplicity assumed that scattering is small compared to thegain. It follows that the scattering losses can become high already at asmall value of the ratio between stimulated emission and scattering (σ₂^(scatter)/σ₂ ^(e)) if r_(effective)≈100, i.e., in a core-doped,cladding-pumped device. Then, already a value (σ₂ ^(scatter)/σ₂ ^(e)) aslow as {fraction (1/1000)} can result in significant losses. Incontrast, in ring-doped cladding-pumped devices, acceptable values of(σ₂ ^(scatter)/σ₂ ^(e)) will be one or two orders of magnitude larger.

Next, we will show how ring-doping also can reduce the sensitivity toquenching.

Very often, some active centra in a gain medium are defect. Thesequenched centra retain their ground-state absorption (GSA), but, if theyabsorb a photon, they are not efficiently excited. This leads to aso-called unsaturable absorption, the spectrum of which is approximatelyproportional to the small-signal ground-state absorption spectrum of themedium. For instance, this type of unsaturable absorption has beenobserved in the important Yb³⁺:glass and Er³⁺:glass gain media. Thesmall-signal absorption is given by:

α₂ ^(ss)=σ₂ ^(a) [G ₂(σ₁ ^(a)+σ₁ ^(e))−(Ψ_(2, doped)/Ψ_(1, doped))G ₁(σ₂^(a)+σ₂ ^(e))]/(σ₁ ^(a)σ₂ ^(e)−σ₁ ^(e)σ₂ ^(a))[dB]  (6)

If, for instance, 3% of the active centra are quenched, we get anunsaturable absorption of 0.03×α2ss. Equation 6 is very similar to Eq.5, and the same result holds: A cladding-pumped device with thecurrently disclosed design will be typically 10-100 times less sensitiveto quenching than are core-doped designs of the prior-art. (This doesnot apply to four-level systems, for which α2ss=0 dB.)

Next, we consider the case of excited-state absorption at the signalwavelength λ₂. Again, a stronger interaction leads to more power lostthrough ESA, at least for a device with significant small-signalabsorption, as the following equations will show. The excited-stateabsorption can be written as:

α₂ ^(ESA)=σ₂ ^(ESA) [G ₂/σ₂ ^(a)−(Ψ_(2, doped)/Ψ_(1, doped))G ₁/σ₁^(a)]/[(σ₂ ^(e)−σ₂ ^(ESA))/σ₂ ^(a)−σ₁ ^(e)/σ₁ ^(a)][dB]  (7)

For a transition to the ground-state, the total excited-state absorptioncan be significant already for values of σ₂ ^(ESA)/(σ₂ ^(e)−σ₂ ^(ESA))of {fraction (1/1000)}. Again, in cladding-pumped devices, thesensitivity can be reduced one or two order of magnitudes byring-doping. (For four-level transitions, σ₂ ^(a)=0, so the sensitivityto ESA is independent of any ring-doping, and equal to that oftraditional core-doped, core-pumped devices.)

Equations 1-7 thus demonstrate how ring-doping makes the discloseddevices less susceptible to absorption loss and scattering losses and toemission losses to ASE at an undesired, high-gain wavelength. Theimprovements are a direct consequence of the reduction of the effectivearea ratio r_(effective)≠r to values around 1-10. In contrast, inprior-art devices, the signal light in the core is confined to an areaapproximately 100 times smaller than that of the pump, so the arearatios r≈r_(effective)≈100. While the area ratio can well be madelarger, a smaller area ratio is troublesome since a smaller area of theinner cladding can make it difficult to launch the pump into the device,and since a larger signal spot-size leads either to a large bendsensitivity or to a multi-mode core.

In addition to the general designs described up to this point, we nextdescribe some particular cladding-pumped fiber lasers and amplifierswith sizable advantages compared to the prior art.

Ytterbium-doped Fiber Operating in Wavelengths Between 975 and 985 nm

For Yb³⁺-doped devices at these wavelengths, the suppression ofquasi-four-level emission around 1030 nm can be especially troublesomefor cladding-pumped devices designed according to the prior art. For awavelength of 975 nm (corresponding to the peak of the cross-sections)with representative cross-section values (cf. Table 1), Eq. 3 gives thefollowing relation between the gain at 975 nm, the gain at 1028 nm, andthe pump absorption of the pumped (i.e., partly bleached) fiber:

G₁₀₂₈=0.25G ₉₇₅+0.74(Ψ_(doped)/Ψ_(p, doped))α_(p) ^(operating)[dB]  (8)

Here, we have assumed that Ψ_(975, doped)=Ψ_(1028, doped), which is areasonable approximation for guided modes at nearby wavelengths. Now,assume that we want the laser to work at 975 nm, with 3.5% reflectivityat one end and 100% at the other one. Then, if the background losses arenegligible, G₉₇₅=7.28 dB.

Consider a representative core-doped prior-art design withr≈r_(effective)=100. Then, for every dB of pump absorption we get 74 dBof gain at 1028 nm. Since the gain at unwanted wavelengths must be belowapproximately 50 dB, we would have to restrict the single-pass pumpabsorption to below 1 dB or 20%. This would be a highly inefficientlaser.

Instead, we propose to use ring-doping. Then, the pump absorption can be5 dB or more, which allows for a good laser efficiency. Note thatincreasing the end-face reflectivity at 975 nm will not help us much, asthe high gain at 1028 nm largely follows from the requirements on pumpabsorption, while it is comparatively insensitive of the gain at 975 nm.For the same reason, the gain at 1028 nm will not be much higher for ahigh-gain amplifier at 975 nm than it is for a low-gain laser, so thedisclosed design provides benefits for both applications.

A high-power laser at 975 nm can be used for pumping Er³⁺. Also otherwavelengths can be used for this, e.g., 980 nm and 985 nm. However, alsothose wavelengths are severely affected by unwanted emission around 1030nm.

TABLE 1 Cross-sections for absorption and stimulated emission used insome numerical examples. Wave- Absorption Emission Active length/cross-section/ cross-section/ medium nm 10⁻²⁵ m² 10⁻²⁵ m² Remark Nd³⁺:glass 800 20 0 Pump to ⁴F_(5/2). Nd³⁺: glass 870 10 10 Nd³⁺: glass 1050 0 30 Unwanted wavelength Yb³⁺: glass 912 8.25 0.275 Pump Yb³⁺: glass 97525.85 25.85 Yb³⁺: glass 980 6.76 8.57 Yb³⁺: glass 985 1.77 2.97 Yb³⁺:glass 1030  0.45 6.3 Unwanted wavelength Er³⁺: glass 980 2 0 Pump to⁴I_(11/2) Er³⁺: glass 1531  5 5 Er³⁺: glass 1550  2.4 3.8 Er³⁺: glass1564  1.6 3 Unwanted wavelength

In the following, we will show that lasing at 975 nm will beparticularly sensitive to any unbleachable Yb³⁺, the existence of whichhas been reported in [R. Paschotta, J. Nilsson, P. R. Barber, A. C.Tropper, and D. C. Hanna, “Lifetime quenching in Yb doped fibers”,submitted to Optics Communications]. This sensitivity can be order ofmagnitudes higher in core-doped designs according to prior art, comparedto the devices of the present invention.

From Eq. 6, we get

α₉₇₅ ^(ss)=1.07G ₉₇₅+6.48(Ψ_(doped)/Ψ_(p, doped))α_(p)^(operating)[dB]  (9)

Hence, with a prior-art design for cladding-pumping, we get severalthousand decibels of small-signal absorption at 975 nm for a desiredpump-absorption of around 5 dB. For r_(effective=)100, already anunsaturable fraction of 1% of this (the lowest value reported in R.Paschotta, J. Nilsson, P. R. Barber, A. C. Tropper, and D. C. Hanna,“Lifetime quenching in Yb doped fibers”, submitted to OpticsCommunications) leads to an unsaturable absorption of around 30 dB,which is unacceptable. With the new devices, the sensitivity isdrastically reduced. Even further reductions are possible by lasing atother wavelengths, e.g.,

α₉₈₀ ^(ss)=0.83G ₉₈₀+1.34(Ψ_(doped)/Ψ_(p, doped))α_(p)^(operating)[dB]  (10)

at 980 nm, and

α₉₈₅ ^(ss)=0.64G ₉₈₅+0.37(Ψ_(doped)/Ψ_(p, doped))α_(p)^(operating)[dB]  (11)

985 nm. The sensitivity to quenching is much reduced, and can be quitesmall in a ring-doped device.

While the analytic considerations above clearly demonstrate theadvantages of the disclosed devices, they do not quantify the advantagesin terms of the most important laser characteristics, namely, pumpthreshold P_(th) and slope efficiency η_(slope). In order to provide amore complete description of the improvements compared to prior art, wenext present calculations of P_(th) and θ_(slope) from simulations witha spectrally and spatially resolved numerical model [B. Pedersen, A.Bjarklev, J. H. Povlsen, K. Dybdal, and C. C. Larsen, “The design oferbium-doped fiber amplifiers”, J. Lightwave Technol. 9, 1105-1112(1991)]. The only significant simplification in the model is that thepump is always assumed to be uniformly distributed across the innercladding. Besides that, the gain medium is assumed to be homogeneouslybroadened, which is reasonable for Yb³⁺:glass systems. With the model,we analyzed fibers of different core-doped and ring-doped designs. Inall cases, we kept the doped area constant, equal to the core size,while the outer radius r_(d) ^(outer) of the doped area and henceΨ_(doped) was varied. The area ratio was 80, and Ψ_(p, doped)=(3080 μm²)⁻¹. Other parameters are given in Table 2.

A first studied cavity had one laser mirror formed by a bare, cleaved,fiber end, providing a broadband reflectivity of 3.5%, while anarrow-band reflector (typically, a fiber bragg-grating) provided a99.9% reflectivity in a desired laser wavelength range 975 nm to 977 nm.Outside this range, the reflectivity was zero, as can be achieved withan AR-coated or an angle-cleaved fiber end.

Lasing in the desired wavelength range was prevented by strong ASE atlong wavelengths (1028 nm−1035 nm) until r_(d) ^(outer) became 5 μm.Then, the diameter of the inner ring is 4.2 μm and r_(effective)=5.8 ingood agreement with earlier estimates. The results are presented inTable 3.

TABLE 2 Values used in detailed Yb-calculations. Other parameters as inTable 1. Quantity Symbol Value Numerical aperture NA 0.1 Core diameter 7μm Cut-off wavelength λ_(c) 915 nm Doped area A_(doped) 38.5 μm² Ybconcentration [Yb³⁺] 2.7 × 10²⁵ m⁻³ Signal overlap with core Γ_(core)0.796 Area of inner cladding A_(pump) 3080 μm² Pump overlap with coreΓ_(p,core) 1/80 Effective area ratio for r_(effective) 63.7 core-dopeddevice Small-signal pump-absorption α_(p) ^(SS) 1.21 dB/m Metastablelifetime τ 0.76 ms Background loss — 0 dB/m Reflectivity, pump launchend — 99.9% at desired wavelength, 0 elsewhere Reflectivity at other end— Either 3.5% broadband, or 50% at desired wavelength and 0% elsewhere

TABLE 3 Laser characteristics of 10 m long unquenched fiber operating at976 nm, with a HR fiber grating and a bare, cleaved end providing thelaser cavity reflections. The small-signal absorption α^(SS) applies toa wavelength of 977 nm. The transmitted pump power P_(p) ^(transmitted)is expressed as a fraction of the launched pump power. r_(d) ^(inner) isthe inner radius of the gain medium. r_(d) ^(outer)/ r_(d) ^(inner)/α^(SS)/ η_(slope) × μm μm r_(effective) dB/m P_(th)/W 100 P_(p)^(transmitted) 3.5-5 0-3.6 64-12 170-31 No lasing for a pump 22%-46%power of 5 W. ASE around 1030 nm dominates the output 5.5 4.2 5.8 15.32.01 ± 0.1 69 ± 2 26% 6.0 4.9 2.9 7.47 2.14 ± 0.1 66 ± 2 29% 6.5 5.5 1.64.03 2.37 ± 0.1 62 ± 2 33% 7.0 6.1 1.0 2.58 2.78 ± 0.1 61 ± 2 34% 7.56.6  0.57 1.43 3.62 ± 0.1 51 ± 2 44%

Clearly, in contrast to prior-art devices, the device disclosed here canlase at 976 nm with a good efficiency. The range of acceptable effectivearea ratios is 1-6. The slope efficiency with respect to absorbed pumpwas approximately 93%—a quite high number which in reality will belowered by background losses. These were assumed negligible in thecalculations.

A shorter fiber length favors lasing at shorter wavelengths in atwo-level system like this. However, shortening the fiber to 5 m is notsufficient for lasing at 976 nm in a core-doped design. Moreover, atthis length, a significant fraction of the pump is not absorbed. Hence,making the fiber sufficiently short to ensure 976 nm lasing in acore-doped design is not an attractive option, even if the pump isdouble-passed through the cavity. The conclusion is that prior-artdesigns are inadequate for lasing at 976 nm for the considered arearatio.

Above, the smaller ring diameters appear to be better than the largerones (provided that lasing is obtained). However, if a fraction of theYb-ions are quenched, this will change, as is evident from Table 4.

TABLE 4 Laser characteristics of 5 m long fiber operating at 976 nm,with 2% of the Yb³⁺-ions quenched. A HR fiber grating and a bare,cleaved end provided the laser cavity reflections. r_(d) ^(outer)/ r_(d)^(inner)/ α^(SS)/ η_(slope) × μm μm r_(effective) dB/m P_(th)/W 100P_(p) ^(transmitted) 3.5-4.5 0-3.5 64-22 170-57 No lasing for pump33%-47% power of 10 W. ASE around 1030 nm dominates the output 5.0 3.512 30.6 1.69 ± 0.1 29 ± 2 50% 5.5 4.2 5.8 15.3 1.77 ± 0.1 34 ± 2 53% 6.04.9 2.9 7.47 2.04 ± 0.1 33 ± 2 58% 6.5 5.5 1.6 4.03 2.57 ± 0.1 29 ± 264% 7.0 6.1 1.0 2.58 3.58 ± 0.1 26 ± 2 68%

At 980 nm and 985 nm, the fiber behaved similarly as at 976 nm, exceptthat 985 nm would not lase for an output reflectivity of 3.5%. A gratingwith 50% reflectivity at the output end allowed for lasing at 985 nm. Incontrast, lasing at 976 nm in an unquenched fiber was only marginallyimproved by a grating also at the output end, and for a partly quenchedfiber, results were worse with a grating than with a bare end. Also, aspredicted in Eqs. 9-11, the longer wavelengths are less sensitive toquenching than are the976 nm lasers.

These and other detailed numerical model calculations have shown:

The earlier analytic considerations are largely accurate in determiningwhether or not a laser can work efficiently.

The disclosed devices perform much better as lasers at 975 nm-985 nmthan do prior-art designs.

The best value of the effective area ratio is around 3-10 for thislaser.

The sensitivity to quenching is reduced with a smaller effective arearatio.

The susceptibility to quenching is smaller at 980 nm and especially at985 nm than it is at 976 nm.

Neodymium-doped Fiber Operating on the ⁴F_(3/2)→⁴I_(9/2) Transition (850nm-950 nm)

A device designed in a similar way as the Yb³⁺-doped cladding-pumpedfiber will also improve on prior-art designs for this Nd³⁺-transition.For Nd³⁺-doped devices at these wavelengths, the suppression of thedominant ⁴F_(3/2)→⁴I_(11/12) at 1050 nm transition is a problem,especially for cladding-pumped devices. For a wavelength of 870 nm,typical cross-sections (cf. Table 1) gives the following relationbetween the gains at 870 nm and around 1050 nm and the pump absorptionof the pumped fiber:

G ₁₀₅₀=3G ₈₇₀+1.5(Ψ_(doped)/Ψ_(p, doped))α_(p) ^(operating)[dB]  (12)

The relations will be similar for other wavelengths in this transition.Equation 12 reveals that the gain at 1050 nm will be at least threetimes larger than that at 870 nm. This limits the 870 nm gain to 15 dB—acomparatively low but still useful number. However, with a prior-art,core-doped device, it will not be possible to absorb the pump properly,since the gain at 1050 nm becomes prohibitively high already for asingle-pass pump absorption of less than 0.5 dB (≈10%). On the otherhand, in a ring-doped device, r_(effective) can be reduced by a factor10 or more, so an absorption α_(p) ^(operating) of at least 5 dB (=68%)is possible, while still allowing for a single-pass gain at 870 nm of 10dB.

In Eq. 12, we for simplicity assumed that Ψ_(doped) is equal at 1050 nmand 870 nm. However, for ring-doping, Ψ_(doped) will be larger at 1050nm than at 870 nm. This means that the factor “3” in Eq. 12 actuallywill be larger. For instance, with a numerical aperture of 0.1 and acore diameter of 6 μm, a doped ring with r_(d) ^(inner)=4 μm and r_(d)^(outer)=5 μm gives (Ψ_(1050, doped)/Ψ_(870, doped))=1.6. Then,G₁₀₅₀=4.8 G₈₇₀+1.5(Ψ_(doped)/Ψ_(p,doped))α_(p) ^(operating).Nevertheless, appropriate designs allow enough gain for efficient lasingat 870 nm before the gain at 1050 nm becomes unrealistically large. The870 nm gain can be even higher in modified designs: If the core has ahigher cut-off wavelength of, e.g., 950 nm, the core will be multi-modedat 870 nm. Since the higher-order LP₁₁-mode penetrates further into thecladding than the fundamental LP₀₁-mode does, the LP₁₁-mode gain at 870nm is higher than the gain of the LP₀₁-mode. Hence, higher-order modelasing at 870 nm becomes relatively easier to achieve compared to the1050 nm lasing in the fundamental mode.

Erbium-doped Fiber Operating on the ⁴I_(13/2)→⁴I_(15/2) Transition (1450nm-1600 nm)

The concerns of this device are similar to those of the cladding-pumpedYb-doped fiber described above. For instance, if we want the device tooperate at 1531 nm, emission at 1564 nm or longer wavelengths is apotential problem in an aluminosilicate host. From Eq. 3 and Table 1, weget

G ₁₅₆₄=0.6G ₁₅₃₁+0.7 (Ψ_(doped)/Ψ_(p, doped))α_(p)^(operating)[dB]  (13)

Clustering is a well-known problem in erbium-doped fibers, and resultsin a saturable absorption. Equation 6 gives

α₁₅₃₁ ^(ss) =G ₁₅₃₁+5 (Ψ_(doped)/Ψ_(p, doped))α_(p)^(operating)[dB]  (14)

These numbers are similar to the ones for Yb³⁺ operating at 976 nm, soring-doping allows for similar improvements as for Yb³⁺.

The wavelength range 1550 nm-1565 nm is technologically important foroptical communication systems. In this range, lasing at 1550 nm may beparticularly hard to achieve, because the gain at, e.g., 1564 nm maybecome prohibitively large. From Eq. 3, we get

 G ₁₅₆₄=0.79G ₁₅₅₀+0.15(Ψ_(doped)/Ψ_(p, doped))α_(p)^(operating)[dB]  (15)

Also in this relatively benign case, adequate pump absorption can betroublesome in a prior-art design for unfavorable values ofr_(effective)≡(Ψ_(doped)/Ψ_(p, doped)), so a ring-doped fiber will beadvantageous. As it comes to the unsaturable absorption, we have that

α₁₅₅₀ ^(ss)=0.6G ₁₅₅₀+2.0(Ψ_(doped)/Ψ_(p, doped))α_(p)^(operating)[dB]  (16)

Core doped devices can then have a small signal absorption of 1000 dB.Even an unsaturable fraction as low as 1% of this small signalabsorption will create an unacceptable unsaturable loss of 10 dB.Consequently we conclude that ring-doped designs are better.

Principle

The type of high-energy pulse amplifiers and lasers we consider areso-called energy-storage devices in which a pulse extracts significantamounts of energy stored in the gain medium. The energy supplied by thepump during the generation/amplification of a single pulse can benegligible. The amount of energy stored in the device then sets an upperlimit on how much energy can be extracted by a pulse. This is asignificant difference compared to other laser and amplifiers, for whichpower extraction is typically limited by the supplied pump power, and inany case not by the stored energy.

In order to obtain high-energy pulses from such an energy storage laseror an amplifier, we need both a large stored (and extractable) energyand a sufficiently high gain. While the gain efficiency of waveguidingamplifiers means that it is often easy to meet the second objective, thesame gain efficiency can make it difficult to store large amounts ofenergy in the device: The gain efficiency implies that a comparativelysmall amount of extractable energy in the gain medium leads to a highgain. However, as already pointed out, since ASE limits the achievablegain of the device, it also limits the energy that can be stored [J.Nilsson and B. Jaskorzynska, “Modeling and optimization of lowrepetition-rate high-energy pulse amplification in cw-pumpederbium-doped fiber amplifiers”, Opt. Lett. 18, 2099-2101 (1993).].

The gain G in a transverse mode is related to the energy E stored in thegain medium through the following relation:

Here, hv is a photon energy, αL is the unpumped loss of the medium,U_(sat)≡hv/(σ^(a)+σ^(a)) is the saturation energy fluence,E^(extractable) is the energy over the bleaching level, i.e., themaximum energy that can be extracted from the device, andE_(sat)≡U_(sat)/Ψ_(doped) is the saturation energy. The important pointis that G is proportional to Ψ_(doped). Hence, a smaller value ofΨ_(doped) leads to a smaller gain per unit extractable energy.Therefore, for a gain medium located in a region where the normalizedmodal intensity of the signal beam is small, the extractable energy fora given gain will be high. Then, if the gain is sufficiently large forthe device in question, a device with low values of Ψ_(doped) will becapable of generating or amplifying pulses to high energies.

Here, we disclose the use of devices that, although the light is tightlyconfined in a single- or few-moded waveguide, have a small value ofΨ_(doped) for high-energy pulse amplifiers and lasers, e.g. Q-switchedand gain-switched ones. Note that any effect this may have on therelative gain at different wavelengths can be counteracted by simplymaking the device longer or increasing the concentration of activecentra.

In addition to the general geometries described earlier, we will nowdescribe some specific geometries and devices.

Core-pumped Ring-doped Pulse Fiber Amplifier or Fiber Laser

In the important class of core-pumped devices, the pump and the signalare guided by the same core. For instance, most erbium-doped fiberamplifiers (EDFAs) are of this type. Typically, the gain medium can be aTm³⁺, Sm³⁺, Ho³⁺, Nd³⁺, Er³⁺, or Yb³⁺-doped glass. The desire weaknessof the interaction between the signal beam and the gain medium normallythen implies that also the interaction with the pump beam is weak,whereby the pumping of the medium becomes weaker and the pump absorptionsmaller. Nevertheless, the disclosed devices can show significantimprovements.

We can distinguish two cases:

1. The pump and signal wavelengths are close, so the signal and pumpmode profiles are close to each other. In this case, it is just a matterof finding suitable values of Ψ for placing the ring. These will dependon the lifetime and cross-sections of the dopant, the pump power andpulse energy, and other parameters. FIG. 6 shows how the extractableenergy and small-signal gain at 1550 nm depends on the position of thering for a ring-doped EDF core-pumped by 0.1 W, 0.2 W, and 0.5 W at 1480nm. FIG. 6 illustrates the extractable energy and small signal gain at1550 nm for a ring-doped erbium-doped fiber (EDF) pumped by 0.1 W, 0.2W, and 0.5 W at 1480 nm in the core. The ring thickness was sufficientlythin to make variations of the normalized intensity of the modal fieldnegligible over its thickness. Other parameters are listed in Tables 5and 6 under “normal core” amplifier and a “large core” amplifier. In allcases, a higher pump power gives a higher small-signal gain and a largerextractable energy. Moreover, the fiber length was optimized for maximumsmall-signal gain in all cases. The advantages compared to the prior-artEDFs (also shown) are substantial. FIG. 8 shows model calculationresults on how Ψ_(doped) depends on the ring position for the ring-dopedEDF. The method used for these and other similar calculations in thisspecification follows [J. Nilsson and B. Jaskorzynska, “Modeling andoptimization of low repetition-rate high-energy pulse amplification incw-pumped erbium-doped fiber amplifiers”, Opt. Lett. 18, 2099-2101(1993).].

2. The pump and signal mode profiles are different. In this case, thepump is unlikely to penetrate far into the cladding, so the doped regionmust be inside the core or immediately outside the core. Unfortunately,for positions for which the signal intensity is suitable, the pumpintensity tends to be much too weak. A good design should then aim atreducing this problem as far as possible. FIG. 7 is similar to FIG. 6,except that the pump wavelength is now 980 nm. In particular, FIG. 7illustrates the extractable energy and small-signal gain at 1550 nm fora ring-doped erbium-doped fiber (EDF) pumped by 0.1 W, 0.9 W and 0.5 Wat 980 nm in the core. The ring thickness was sufficiently thin to makevariations of the normalized modal intensity negligible over itsthickness. Other parameters are listed in Tables 5 and 6 under “normalcore amplifier”. For comparison, also results for EDFAs homogeneouslydoped throughout the core are shown, both for the “normal core”amplifier and a “large core” amplifier. In all cases, a higher pumppower gives a higher small-signal gain and a larger extractable energy.Moreover, the fiber length was optimized for maximum small-signal gainin all cases. We see that the results are now worse, and that thebenefits of ring-doping are smaller. However, performance is stillsuperior compared to that of prior-art designs.

TABLE 5 Geometrical and dopant parameters for energy-storage EDFAs.Normal-core Large-core Quantity Symbol amplifier amplifier Core diameter5 μm 11 μm Numerical aperture NA 0.171 0.100 Cut-off wavelength λ_(c)1118 nm 1437 nm Signal overlap with core Γ_(core) 0.651 0.795 Area ofinner cladding for cladding- A_(pump) 1571 μm² 1571 μm² pumping Pumpoverlap with core for cladding- Γ_(p,core) 1/80 1/16.5 pumping Effectivearea ratio for core-doped — 52.1 13.1 device Background loss — 0 dB/m 0dB/m

TABLE 6 Spectroscopic parameters for energy-storage EDFAs. QuantitySymbol Value Metastable lifetime τ 10.9 ms Absorption cross-section at1480 nm σ^(a) ₁₄₈₀ 1.87 × 10⁻²⁵ m² Emission cross-section at 1480 nmσ^(e) ₁₄₈₀ 0.75 × 10⁻²⁵ m² Absorption cross-section at 980 nm σ^(a) ₉₈₀  2 × 10⁻²⁵ m² Absorption cross-section at 1550 nm σ^(a) ₁₅₅₀ 2.45 ×10⁻²⁵ m² Emission cross-section at 1550 nm σ^(e) ₁₅₅₀ 3.83 × 10⁻²⁵ m²Pump intensity required at 1480 nm I_(sat) 0.0470 mW/μm² to invert 35.7%of the population Pump intensity required at 980 nm I_(sat) 0.0930mW/μm² to invert half the population

As an alternative, the core can be single-mode at the signal wavelength,and multi-moded for the pump. It is well-known that pump-light inhigher-order modes will penetrate further into the cladding, therebyimproving the pumping of the gain medium. Moreover, for so-calledupconversion devices, the pump wavelength is shorter than the signalwavelength, with the favorable side-effect that the pump extends furtherinto the ring, even if it is in the same mode as the signal.

FIG. 9 shows measured results on high-energy pulse amplification for aring-doped, core-pumped Yb³⁺-doped fiber amplifier according to thepresent embodiments. FIG. 9 illustrates the extractable energy (“pulseenergy above cw”) vs launched pump power for a core-pumped fiberamplifier with an Yb³⁺-doped ring. The fiber was pumped at 1000 nm, andamplified signal pulses at 1047 nm. The highest recorded extracted pulseenergy (above the cw-level) of more than 60 μJ can be compared topublished 10 μJ total pulse energy from large-area core amplifier(albeit at a lower pump power of 160 mW) [D. T. Walton, J. Nees, and G.Mourou, “Broad-bandwidth pulse amplification to the 10-μJ level in anytterbium-doped germanosilicate fiber”, Opt. Lett. 21, 1061-1063(1996)], as used in the prior art for high pulse-energies. Thering-doped fiber had Ψ_(doped)≈0.02 μm². A smaller value can allow foreven larger extracted energies, as long as the pump power is largeenough to create a significant gain.

The emission cross-section of erbium in glass is smaller than for manyother gain media, like Nd³⁺:glass at 1050 nm and many transition metals.It follows from Eq. 17 that the stored energy will be smaller in thesemedia. Therefore, the improvements with ring-doping can be relativelylarger than for Er³⁺:glass.

Cladding-pumped Devices

We now describe cladding-pumped ring-doped fibers for high-energy pulseamplification and generation. Because of the typically higher pumppowers used with these devices and because of the separatelycontrollable normalized pump and signal mode intensities in the dopedregion, the disclosed cladding-pumped devices will by far outperform anyprior-art core-doped single- or few-moded waveguiding device. A typicaldevice will be a rare-earth-activated glass fiber optically pumped by apump beam launched into the inner cladding (cf. FIG. 3).

FIG. 10 shows how the extractable energy and small-signal gain at 1550nm depends on the position of the ring for a ring-doped EDFcladding-pumped by 1 W and 5 W at 980 nm. In particular, FIG. 10illustrates the extractable energy and small-signal gain at 1550 nm fora ring-doped EDF cladding-pumped by 1 W and 5 W at 980 nm. The ringthickness was sufficiently thin to make variations of the normalizedmodal intensity negligible over its thickness. Other parameters arelisted in Tables 5 and 6 under “normal-core amplifier”. For comparison,also results for EDFAs homogeneously doped throughout the core areshown, both for the “normal core” amplifier and a “large core”amplifier. In all cases, a higher pump power gives a higher small-signalgain and a larger extractable energy. Moreover, the fiber length wasoptimized for maximum small-signal gain in all cases. In all cases, thefiber length was optimized for maximum small-signal gain. Otherparameters were the same as in FIG. 10, and are listed in Tables 5 and6. The advantages compared to the prior-art EDFs (also shown) aresubstantial. The increase of the extractable energy can approach twoorders of magnitude in the devices studied here.

In view of these results, we propose a ring-doped cladding-pumpedoptical fiber where the ring is located at a position where the modeintensity is, e.g., one or two orders of magnitude smaller than it is inits center. In order to get sufficient absorption, sensitization can beused, e.g., as in ytterbium-sensitized erbium-doped fibers.

A ring-shaped gain medium outside the core can be better pumped by abeam in the cladding. Thus, while cladding-pumping has normally beenconsidered to facilitate launching of non-diffraction-limited sourceslike diode bars, we also propose to use cladding-pumped, ring-dopedfibers even when high-brightness, near-diffraction-limited pumps thatcould be efficiently launched into the core are available. The highbrightness will still be favorable because the area of the innercladding can be small. In these devices, cladding-modes can see a highergain than the desired core-mode does, whereby some measure forsuppressing cladding-modes would be required.

Even if only a small part of the stored energy is extracted from aring-doped amplifier, the high stored energy can still be advantageous,since it for instance reduces the distortions of the chirped-pulseamplification with small distortions of the pulse shape.

Passively Q-switched and Gain-switched Lasers

In passively Q-switched lasers, energy and thereby ASE builds up in aregion of gain. The ASE then transfers energy to a saturable absorber.The saturable absorber must be so that the absorption change per unitstored energy is smaller than it is in the gain region. In prior-artdevices, this is achieved by using a saturable absorber with largeabsorption and/or stimulated emission cross-sections, compared to thoseof the gain medium. Ring-doped fibers open up for Q-switched laserswhere the gain section and the saturable absorber are made from the samematerial, e.g. an erbium-doped glass. This is possible sinceE_(sat)≡hv/[ψ_(doped)(σ^(a)+σ^(e))] can be two orders of magnitudehigher in the ring-doped fiber than in the core-doped one, even thoughthe material-dependent quantities (σ^(a)+σ^(e)) are equal in the twodifferent fibers.

The gain section can also be a core-doped fiber with a large area core,however, this does not work as well as a properly designed ring-dopedfiber.

In a first embodiment, a ring-doped fiber is cascaded with a core-dopedfiber, each of which are doped with a similar dopant with anon-negligible ground-state absorption, to form a laser cavity. A pumpbeam is launched into a gain section, consisting of the ring-dopedfiber, thereby building up a gain and stored energy. A cw pump beam canbe used, and the fiber can be cladding-pumped. The gain sectiongenerates ASE, through which energy is transferred from the gain sectionto a core-doped fiber constituting a saturable absorber. The pump alsoacts to bleach the pump-absorption in the ring-doped fiber, whereby thepump penetrates deeper into the cavity, and possibly helps in bleachingthe saturable absorber. The transfer of energy from the gain section tothe absorber section increases the net gain in the cavity to a pointwhere it exceeds threshold. Then, energy is radiated from the cavity inthe form of a Q-switched pulse. This substantially reduces the storedenergy, and hence the gain, in the cavity, so that the ASE becomesnegligible, and the pump power that penetrates to the saturable absorberbecomes small. The saturable absorber then relaxes to a state that is atleast partly absorbing. Thereby, the absorption in the saturableabsorber has increased substantially before the gain section starts togenerate ASE again, whereupon the cycle is repeated.

A second embodiment is similar to the first embodiment, except thatthere is provided a pump-absorber or a pump reflector between thegain-section and the saturable absorber. This substantially reduces thepumping of the saturable absorber.

A third embodiment is similar to the first or second embodiment, exceptthat the active centra in the gain medium and the saturable absorber aredifferent. The pump wavelength can be chosen so that it cannot bleachthe saturable absorber.

FIG. 11 is a view of a fiber having a saturable absorber (640) in thecentral part of the core (30), and a ring-shaped gain medium (620)around the absorber. In the illustrated example, the gain medium residesin the core, but it can be placed partly or wholly in the cladding (10).

FIG. 12 illustrates a semiconductor amplifier for signal amplification.The semiconductor amplifier provides gain for a guided optical signalbeam in a region where the normalized modal intensity is small andcomprises a cladding (410), a gain region or an active layer (420), acore or index guiding layer (430), a substrate (480), and a contactlayer (490). Also the approximate location of a signal beam is indicated(470). The refractive index of the active layer (420) can be depressedwith respect to the remainder of the cladding (410) in order to suppressgain guiding.

FIGS. 13a to c illustrate devices in which unwanted, higher-order modesare suppressed by the inclusion of an absorber. FIG. 13a shows a fiberwith an amplifying ring 10 and an absorbing ring 510 configured tosuppress high-order modes. The absorption of the desired fundamentalmode is small or even negligible. FIG. 13b shows a planar waveguide withamplification of the evanescent field by a gain region 120, within anabsorbing superstructure 520. Again, the absorption of the desiredfundamental mode is small or even negligible. Undesired higher-ordermodes penetrate further into the absorber, whereby they are suppressed.FIG. 13c shows a double clad ring-doped fiber in which asignal-absorbing region 530 has been incorporated into the cladding,thereby preventing any build-up of signal light in the cladding.

Device with a Distributed Saturable Absorber

Above, two media with different saturation characteristics were combinedin a cascade. However, the two gain media can also reside side by sidein the same fiber. An example of this is illustrated in FIG. 11. A fiberhaving a core (30) and a cladding (10) is doped with a saturableabsorber (640) and a gain medium (620). Here, the saturable absorber islocated in a region where the normalized modal intensity is larger thanit is in the region of the gain medium. Hence, if the absorber and gainmedia are similar (except that the gain medium is pumped), and thecross-section for stimulated emission of the gain medium is similar tothe absorption cross-section of the absorber, the small-signal gain ofthe fiber can be negative or small, even though the extractable energyof the gain medium is larger than the energy required to bleach thesaturable absorber. Hence, the ASE in the fiber can be suppressed, whilethe energy that can be extracted from the device, if for instance asignal pulse is launched into it, can be large.

In contrast to the prior art, a ring-shaped gain medium allows theactive centra in the absorber and the gain media to be of the same orsimilar types, as long as it is possible to pump the centra in the gainmedium while leaving those in the absorber medium unpumped. A particularstudied embodiment consisted of an Er³⁺-doped saturable absorber and anYb³⁺-sensitized Er³⁺-doped gain medium. The Er³⁺ in the gain medium wasexcited indirectly (i.e., via the Yb³⁺) by an optical pump beam launchedinto the fiber core. The launched pump power was 1 W at a wavelength of1064 nm, which is a wavelength that will not excite the Er³⁺ in thesaturable absorber. The fiber had a numerical aperture of 0.16, and acore diameter of 7 μm. The diameter of the saturable absorber (640) was1 μm, while the inner and outer radii of the ring-shaped gain medium(620) were 3.4 μm and 3.5 μm, respectively. The Er³⁺-concentration was2.38×10²⁵ m⁻³ in both the absorber and gain media, and theYb³-concentration was 2.97×10²⁶ m⁻³ in the gain medium. The absorptionand emission cross-sections at the peak (wavelength 1536 nm) were both6.8×10⁻²⁵. Hence, the small-signal absorption at that wavelength was 2.1dB/m in the saturable absorber, and 1.3 dB in the (unpumped) gainmedium. Moreover, at 1064 nm, the cross-sections for stimulated emissionand absorption of the Yb³⁺-ions were at 2×10⁻²⁶ m² and 5×10⁻²⁸ m²,respectively. The metastable lifetimes of the Er³⁺ and 10.2 ms and 1.3ms, respectively. The energy was transferred from the Yb³⁺ to the Er³⁺with a rate coefficient k_(tr) of 1.05×10⁻²¹ m³/s [J. Nilsson, P.Scheer, and B. Jaskorzynska, “Modeling and optimization of shortYb³⁺-sensitized Er³⁺-doped fiber amplifiers”, IEEE Photon. Technol.Lett. 6, 383-385 (1994).]. The spectral characteristics of the gain andabsorber region followed those for Er³⁺ and Yb³⁺ in a phosphosilicateglass. Numerical calculations, following those in [J. Nilsson and B.Jaskorzynska, “Modeling and optimization of low repetition-ratehigh-energy pulse amplification in cw-pumped erbium-doped fiberamplifiers”, Opt. Lett. 18, 2099-2101 (1993).] and [J. Nilsson, P.Scheer, and B. Jaskorzynska, “Modeling and optimization of shortYb³⁺-sensitized Er³⁺-doped fiber amplifiers”, IEEE Photon. Technol.Lett. 6, 383-385 (1994).] and using the parameters above, showed thatthe extractable energy in this device was approximately 0.6 mJ at 1536nm and 1.1 mJ at 1560 nm.

In the example above, the ring-shaped gain region was thin and hence theextractable energy per unit length small. This implied that the lengthof the fiber became so long (several hundred meters) that backgroundlosses could become important, and the calculated energy, neglectingbackground losses, difficult to achieve. By placing a ring-shaped gainmedium outside the core (where the normalized modal intensity issmaller), its gain can be kept constant while the stored energy in thegain medium is increased (cf. Eq. 17). Hence, the fiber can be shorter.For a cladding-pumped fiber having an inner cladding with a radius of 10μm, a saturable absorber (640) with a radius of 0.5 μm (small-signalabsorption 2.2 dB/m at 1536 nm), and a gain medium (620) with an innerradius of 4.5 μm and an outer radius of 5.5 μm (small-signal absorption3.3 dB/m at 1536 nm), calculations gave an extractable energy of 0.8 mJat 1536 nm and 1.4 mJ at 1560 nm, for a fiber length of 50 m. The fiberlength can be further reduced by using a larger-area gain region (e.g.,a thicker doped ring). The other parameters of the fiber were the sameas above. A problem with this approached is that the preferred hostmaterial for a Yb³⁺-sensitized Er³⁺-doped gain medium (phosphosilicateglass) has a higher refractive index than the preferred cladding (fusedsilica). Hence, some extra measure may be needed to level the refractiveindex of the gain medium with that of cladding.

The calculations have also shown that ASE in the long-wavelength end ofthe ⁴I_(13/2)→⁴I_(15/2) emission spectrum, where the emissioncross-sections become relatively larger compared to the absorptioncross-section, can build up and partly bleach the absorption andcompress the gain. This can be avoided by introducing an unsaturableloss at these long wavelengths. Bending the fiber provides a method formaking the fiber lossy at 1600 nm, while keeping the unsaturable losssmall at 1536 nm. For example, with the fiber parameters above and witha bend radius of 9 mm, the bend-loss is approximately 0.033 dB/m at 1600nm, 0.012 dB/m at 1560 nm, and 0.0061 dB/m at 1536 nm, i.e., it is fivetimes smaller at 1536 nm than at 1600 nm. Another alternative for anunsaturable loss at longer wavelengths is to use an unsaturable absorberin addition to the saturable absorber. For this particular transition,Tm³⁺:glass and Tb³⁺:glass are suitable systems for an optical fiber, asthe absorption of suitable pump wavelengths (e.g., 1064 nm or at least1047 nm in the case of Tm³⁺) is small, as is the absorption for a signalat 1536 nm. Yet another alternative is to use different host media forthe gain and the absorber media. A suitable host medium for the absorbermakes its spectrum wider, and can thus prevent the build-up of ASE atlong wavelengths.

In the example above, a pump wavelength of 1064 nm was assumed. Otherwavelengths are also possible. However, the pump should not pump theEr³⁺ directly, since then also the saturable absorber will be excited.Moreover, for pumps on the short-wavelength side of the Yb³⁺ absorptionpeak, emission around 980 nm from the Yb³⁺ can build up in the fiber andbleach the Er³⁺-ions in the absorber.

Even if the centra providing the gain and the saturable absorption aredifferent, a design according to FIG. 11 can improve to prior-artdevices in that the gain efficiency of the gain medium is relativelylower than it otherwise would be.

Saturable Absorber

The saturation power P_(sat) of a saturable absorber is given byP_(sat)≡hv/[ψ_(doped)(σ^(a)+σ^(e))τ], where τ is the lifetime of ametastable state. For some devices, a medium that would otherwise be asuitable saturable absorber (e.g., because of a suitable spectralresponse) is inappropriate because its saturation power is too small.This can be the case for an EDF saturable absorber, with P_(sat)typically smaller than 1 mW. We here disclose that ring-doping allowsψ_(doped) to be chosen so that a larger, predetermined value of P_(sat)can be obtained. For this application, a few-moded fiber can beacceptable for single-mode applications, as higher-order modes willexperience a higher loss which can render the power in them negligible.

Signal Amplifiers for Reduced Cross-talk

In some optical amplifiers, especially semiconductor ones, even theenergy of a single signal bit (e.g., 0.1-100 fJ) can be non-negligiblecomparable to the stored energy. Then, already the amplification of asingle bit extracts enough energy to reduce the gain. This leads tofour-wave mixing and cross-talk in multi-wavelength amplifiers andinter-symbol interference in single-wavelength amplifiers. This can beavoided with the higher stored energy that, for a given gain,accompanies the reduced interaction in the devices disclosed in thisinvention.

FIG. 12 illustrates an embodiment. A semiconductor amplifier providesgain for one or several guided optical signal beams in a region wherethe normalized modal intensity is small. The device can be electricallypumped. The refractive index of the gain region can be depressed inorder to suppress gain-guiding, since this can otherwise occur insemiconductor optical amplifiers in which the gain per unit length islarge. This would lead to a large normalized modal intensity in thegain-region, thereby preventing substantial reductions of theinteraction.

Suppression of Unwanted Modes

Often, lasing on a specific transverse mode is desired, and thennormally on the fundamental mode of the core. If so, it may be necessaryto suppress other, undesired, modes. Higher-order guided modes of thecore extend further into the cladding and thus see a significantlyhigher gain than does the fundamental mode in a ring-doped device.Although we normally envisage single-moded cores as preferred designs,higher-order modes can also be present due to fabrication errors, etc.However, these modes are less strongly guided and will be more sensitiveto bending. Hence, with a fiber, simply bending it can reinstate a netgain advantage for the fundamental mode.

Another alternative is to incorporate a region outside the gain regionthat absorbs the signal (at desired and possibly also at undesiredwavelengths) but has a low loss for the pump. This absorbing region islocated so that it preferentially absorbs light in undesired modes.These can be higher-order modes of the core, and also cladding-modes.See FIG. 13.

Several possibilities exist for creating the absorbing region. In thecase of a Yb-doped device, Pr³⁺ and Er³⁺ can be suitable such absorbers.For Nd³⁺ at 850 nm-950 nm, Yb³⁺ can be used. For Er³⁺, Tm³⁺ and Sm³⁺ arepotential candidates, just to mention some possibilities with rare-earthdoping. Sm³⁺ can also suppress unwanted 1050 nm radiation in Nd³⁺-dopedsamples. Optionally, some additional measures can be taken to quench thedopant, to prevent it from bleaching.

Amplifying Optical Devices

FIG. 14 shows an amplifying optical device 1400 comprising a firstwaveguiding structure 1401 comprising a first core 1402 and a cladding1403, and configured to guide optical radiation 1404; a pump source 1405configured to supply optical pump power 1406, an amplifying region 1407situated in the cladding 1403; wherein the pump source 1405 is opticallycoupled to the amplifying region 1407, and wherein in use the opticalradiation 1404 guided in the first waveguiding structure 1401 overlapsthe amplifying region 1407.

Also identified in FIG. 14 is an amplifying optical waveguide structure1408 which comprises the first waveguiding structure 1401 and theamplifying region 1407. The purpose of the amplifying optical device1400 is to generate or amplify optical radiation 1404.

The amplifying optical waveguide structure 1400 can also operate with anamplification less than unity for radiation at certain wavelengths(especially in the absence of optical pump power 1406) and can then beused to absorb optical radiation 1404.

The amplifying optical waveguide structure 1408 has first and secondends 1409 and 1410. The first waveguiding structure extends to first andsecond ends 1409 and 1410, so that optical beams can be coupled into andout of the first waveguiding structure 1401. It is also possible tocouple light into the first waveguiding structure 1401 through the sideof the amplifying optical waveguide structure 1408. Points at whichoptical beams can enter or exit an optical waveguiding structure can bereferred to as input and output ports, as the case may be. An opticalbeam launched through a port can exit through the same port, if, forinstance, the amplifying optical device is a reflecting traveling-waveamplifier.

FIG. 14 illustrates the optical radiation 1404 and the optical pumppower 1406 with the intensity profile in a cross-section of the beams.

By amplifying optical device 1400 we mean a device for generating,amplifying, or absorbing the optical radiation 1404. By way of exampleonly and without limitation, an amplifying optical device 1400 can be anoptical amplifier, a master oscillator power amplifier (MOPA), anamplified spontaneous emission (ASE) source, a superfluorescent source,an energy storage device, a high-pulse energy device, a cladding-pumpeddevice, a semiconductor signal amplifier, or a laser which by way ofexample and without limitation can include a laser, a fiber laser, aQ-switched laser, a mode-locked laser, or a semiconductor laser.

By a pump source 1405, we mean a device for supplying optical pump power1406. By way of example only and without limitation, a pump source 1405can include a gas laser, a solid state laser, a semiconductor laser, achemical laser or a semiconductor light emitting diode. Thesemiconductor laser can be implemented with a diode bar or abroad-stripe laser diode, and can be used either for end-pumping or forside-pumping. A pump source 1405 can also be provided by naturalillumination, for example by daylight. The preferred pump source 1405 isa high-power semiconductor laser diode.

Though FIG. 14 shows a single pump source 1405, multiple pump sourcescan be employed in order to obtain higher powers and/or for pumpredundancy. The pump power 1406 from the multiple pump sources can becoupled into the amplifying region 1407 via at least one of the firstend 1409, the second end 1410 and the side.

Though FIG. 14 shows optical pump power 1406 being launched through afacet at a first end 1409 of the amplifying optical waveguide structure,other schemes for launching the power are also possible, as will bedescribed in this document.

By first core 1402, we mean the region of the first waveguidingstructure 1401 where the intensity of the optical radiation 1404 isrelatively high compared to the intensity of the optical radiation 1404propagating in the cladding 1403 in the same transverse section. By wayof example only and without limitation, the first core 1402 can be thatregion with a refractive index greater than the refractive index of thecladding 1403. However, for the purposes of this invention, the firstcore 1402 may sometimes be defined as the region of the firstwaveguiding structure 1401 bounded by a contour of equal opticalintensity of the fundamental mode and which contains 75% of the opticalradiation 1404 at a wavelength corresponding to the second mode cut-offof the first waveguiding structure 1401. Note however that the devicecannot be operated at the wavelength corresponding to the second modecut-off of the first waveguiding structure 1401.

The first waveguiding structure 1401 can be a single mode waveguidingstructure, or can support several higher-order modes. The firstwaveguiding structure 1401 can be a planar waveguiding structure or canbe an optical fiber.

Though not shown in FIG. 14, it is possible that a waveguiding structurehas branches so that a light beam propagating in one waveguidingstructure is divided into two beams propagating in different waveguidingstructures. Correspondingly, two beams can be combined to one.

In some instances, the propagation of high-order modes, which can beleaky, is undesirable. These can be suppressed by bending the firstwaveguiding structure 1401, or by introducing a absorber into thecladding 1403 configured such that there is a high-differential lossbetween the undesired high-order modes and the desired propagating modeor modes.

The amplifying region 1407 can contain at least one rare earth dopantselected from the group consisting of Ytterbium, Erbium, Neodymium,Praseodymium, Thulium, Samarium, and Holmium. It can also containEuropium, Terbium, and/or Dysprosium. The amplifying region 1407 cancontain at least one transition metal.

For embodiments where the amplifying region 1407 is located in both thefirst core 1402 and the cladding 1403, the rare-earth dopant within theportion of the amplifying region 1407 residing in the cladding 1403 isnot be the same as the rare-earth dopant in the first core 1407.

It can also be that the amplifying region 1407 amplifies opticalradiation 1404 at different wavelengths using, e.g., differentrare-earth dopants, with at least two beams of optical radiation 1404 atdifferent wavelengths propagate simultaneously or alternately throughthe waveguide. Therefore, we take the location of the amplifying region1407 to be determined by the properties of the amplifying region at thewavelength of optical radiation 1404 of interest.

The amplifying region 1407 can be excluded from the first core 1407leading to a so-called ring-doped design. A particular advantage of thisembodiment is that the amplifying region 1407 in operation will containa significantly larger stored energy than corresponding core-dopeddesigns. This can be important for devices that emit optical pulses withpulse energies being a significant fraction of the energy stored in theamplifying region 1407. This can also be important for amplifiers withhigh requirements on linearity, for example in analog Community AntennaTelevision (CATV) applications—especially if the signal containslow-frequency components and the output power is high whereuponundesirable non-linear distortion can occur.

The first waveguiding structure 1401 can be configured such that in usethe first waveguiding structure 1401 is a single-mode waveguide, and theoptical radiation 1404 guided by the first waveguiding structure 1401has a Gaussian equivalent spot size (1/e² intensity diameter) greaterthan about eight times the wavelength as measured in vacuum of theoptical radiation 1404 guided by the first waveguiding structure 1401.

The first waveguiding structure 1401 can be of a more complicated shapethan the traditional ones illustrated in the drawings. For example, itcan be non-circular or utilize complicated core designs such as found inW-fibers, multiple cladding fibers (including those with areas in thecladding with a raised refractive-index), segmented core designs, andso-called alpha profiles.

The amplifying optical device 1400 can be advantageous in several waysas explained previously. For example, in order to incorporate a largeenough amplifying region 1407 to absorb enough pump power someamplifying optical waveguide structures 1408 need to be long, e.g. 0.1kilometers up to several kilometers. Around a wavelength of 1000 nm,low-loss first waveguiding structures made from doped silica fibersexhibit a background loss of at least approximately 1 dB/km. A 1 dBbackground loss may be acceptable in an amplifying optical device 1400.However, an amplifying region 1407 incorporated into the first core 1402tends to significantly increase the background loss, often by more thanone order of magnitude. This reduces the maximum acceptable lengths ofthe amplifying optical waveguide structure 1408, which can have animpact on the pump absorption. By placing the amplifying region 1407 inthe cladding 1403 rather than in the first core 1402 in which most ofthe power of the optical radiation 1404 propagates enables us to use apassive first core 1402 (e.g. made from a pure silica or a silica dopedwith at least one of fluorine, germanium, phosphorus, tantalum,aluminium and titanium). The background losses of the first waveguidingstructure 1401 can then be reduced so that a longer amplifying opticalwaveguide structures 1408 that can absorb more pump power 1406 can beused in order to generate higher power optical radiation 1404.

FIG. 15 shows an amplifying optical device 1500 based on the amplifyingoptical device 1400, which further comprises a second waveguidingstructure 1501 comprising a second core 1502 and configured to guide theoptical pump power 1406, and wherein the second waveguiding structure1501 contains the amplifying region 1407 and wherein the second core1502 is at least partly formed by at least part of the cladding 1403,and wherein the pump source 1405 is optically coupled to the secondwaveguiding structure 1501.

The first core 1402 can form a part of the second core 1502.

Also identified in FIG. 15 is a cladding-pumped amplifying opticalwaveguide structure 1508 which comprises the first and secondwaveguiding structures 1401, 1501 and the amplifying region 1407.

In some instances, unwanted modes in either the first or secondwaveguiding structures 1401, 1501 can be excited. The higher-order modesin the first waveguiding structure 1401 can be suppressed by bending thefirst waveguiding structure 1401. The higher-order modes in the firstand second waveguiding structures 1401 and 1501 can be suppressed byintroducing an absorber into the cladding 1403 configured such thatthere is a high-differential loss between the undesired modes and thedesired mode or modes. Unwanted modes in the second waveguidingstructure 1501 can also be removed by mode stripping—for example byremoving outer coatings and placing the fiber into high-index fluid.

Examples of the first waveguiding structure 1401 and the amplifyingregion 1407 were described previously with reference to FIGS. 1, 2, 3,4, 11, 12, 13 a, 13 b and 13 c. The amplifying region 1407 is shown asthe ring-doped dopant profile 20 in FIG. 1. As FIG. 14 illustrates, theamplifying region 1407 can comprise at least one doped region ofarbitrary shape.

Waveguiding can also be achieved with a micro-structured design, forexample, the first or second waveguiding structures 1401, 1501 cancontain longitudinally extensive holes.

A preferred design of the amplifying optical device 1500 is one in whichin use the optical radiation 1404 can be guided by the first waveguidingstructure 1401 without coupling to the second waveguiding structure1501.

The cross-sectional area of the second core 1502 can be greater than1000 square microns and smaller than 100,000 square microns.

The second core 1502 is preferably adjacent to a region 1503 having alower refractive index than the second core 1502, the amplifying opticaldevice 1502 being such that the region 1503 provides total internalreflection of the optical pump power 1406. The region 1503 can comprisea vacuum, a gas, a liquid, a polymer or a glass. If the amplifyingoptical waveguiding structure 1508 is an optical fiber, the polymer canbe applied as a coating during the fiber drawing process. The amplifyingoptical waveguiding structure 1508 then forms an example of adouble-clad optical fiber, as illustrated in FIG. 4. A double-cladoptical fiber is a preferred amplifying optical waveguiding structure1508. Alternatively the second core 1502 can be surrounded by a metal ora periodic layer for reflecting light.

It is preferred that the first waveguiding structure 1401 and the secondwaveguiding structure 1501 are fabricated in a single optical fiber.

It is preferred that the first waveguiding structure 1401 is fabricatedfrom at least one glass system, preferably an oxide glass systemselected from the group consisting of silica, doped silica, silicate,and phosphate. The second waveguiding structure 1501 can also befabricated from the at least one glass system. By doped silica we mean(WITHOUT LIMITATION FTC) silica doped with fluorine and/or at least oneof the oxides of the following—germanium, phosphorus, boron, tantalum,titanium, aluminum, tin, where the oxide dopant concentration istypically up to around 10%. By silicate, we mean doped silica where thedopant concentration is greater than about 10%. By phosphate we mean aphosphate compound glass which includes phosphoria with the addition ofother glass forming or modifying agents. In addition, the dopantsincluded in any of the above glass systems can include rare earth andtransition elements.

The amplifying optical devices 1400 and 1500 can also contain limitedamounts of gain medium in the first core 1402 while still retaining thebasic characteristics of a device doped in the cladding 1403. We alsonote that the amplifying optical waveguide structures 1408 and 1508 canbe longitudinally varying, e.g., with a section doped in the first core1402 rather than the cladding 1403. Nevertheless, the advantages of thenovel amplifying optical devices disclosed here remain to the extentthat most of the power transferred to the optical radiation 1404 can betransferred from parts of amplifying region 1407 located in the cladding1403.

Nevertheless, the advantages of the novel amplifying optical devicesdisclosed here remain to the extent that most of the power transferredto (or from) the optical radiation 1404 can be transferred from (or to)parts of the amplifying region 1407 located in the cladding 1403.

In order to obtain an efficient device, it is preferred to locate theamplifying region 1407 close enough to the first core 1402 so that theall of the optical pump power 1406 absorbed by the amplifying region1407 can be transferred to the optical radiation 1404. Otherwise,pump-to-signal power conversion efficiency is reduced.

It is well-known that the shape of the second core 1502 as well as thelocation of the amplifying region 1407 relative to the second core 1502affects the rate at which the amplifying region 1407 absorbs the opticalpump power 1406. In particular, an amplifying region 1407 located nearthe center of a circularly symmetric second core 1502 may fail to absorbthe optical pump power 1406 efficiently. Well-known methods forimproving the pump absorption are to locate the amplifying region 1407off-center, to use a non-circular second core 1502, or to bend theamplifying optical waveguide structure 1508.

For efficient absorption of the optical pump power 1406, it is preferredthat the amplifying region 1407 is transversely disposed to regionswithin the second waveguiding structure 1501 where the intensity of theoptical pump power 1406 is high.

FIG. 15 shows a second waveguiding structure 1501 that confines light inboth directions transverse to the first waveguiding structure 1401 sothat the first and second waveguiding structure 1401 and 1501 areparallel to each other. However, for instance, in a planar structurelike the one illustrated in FIG. 3, the second core 1502 can be quitewide in one direction and effectively only confine light in onetransverse direction. In such a structure, the optical pump power 1406can also propagate at an angle to the first optical waveguidingstructure 1401. In order to ensure sufficient pump absorption, theamplifying optical waveguide structure 1408 can contain severalwaveguiding structures 1401 with amplifying regions 1407. The differentwaveguiding structures 1401 can be coupled to each other in series or inparallel, or can be independent.

FIG. 16 shows a preferred embodiment of an amplifying optical device1600 comprising an optical fiber 1610 containing an amplifying region1407 that is characterized by a dopant concentration 1601, a disposition1602 and a length 1603, and wherein the dopant concentration 1601, thedisposition 1602 and the length 1603 of the amplifying region 1407 arearranged such that the amplifying optical device 1600 amplifies at anoperating wavelength at which there is a significant saturable smallsignal absorption, for example as found in a two- or three-level system.

In a preferred embodiment, the optical fiber 1610 is fabricated fromsilica-based glasses. Several suitable dopants that increase therefractive index but are otherwise optically passive are well-known andinclude germania, phosphorus, alumina and tantalum. These can be usedfor defining the first core 1402. The cladding 1403 also functions as asecond core 1502, and can be formed of pure silica except in theamplifying region 1407 which is formed by doping the cladding 1403 witha rare earth such that a ring is formed around the first core 1402.

It is well-known that co-dopants can be used to increase the solubilityof rare earth dopants in silica. This is otherwise poor. Preferredco-dopants include alumina and phosphorous. The incorporation ofalumina, phosphorous, and/or a rare earth in silica is known to increasethe refractive index compared to pure silica. This increase can besignificant, and depending on the design of the optical fiber 1610, itcan be undesirable. If so, it can be negated by further co-doping theamplifying region 1407 with a index-lowering element like fluorine.Alternatively, we can use a cladding 1403 made from silica doped with anindex-raising agent (e.g., tantalum or one of the other index-raisingelements listed above). This way, the refractive index of the cladding1403 can equal or even exceed that of the amplifying region 1407, evenif this is co-doped with, say, alumina or phosphorous. All of thesemethods for modifying the properties of silica are well-known.

Preferred embodiments of the amplifying optical device 1600 enablesefficient cladding pumping of the amplifying optical device 1600 at thisoperating wavelength compared to corresponding core-doped designsutilizing the same rare earth dopant.

A preferred embodiment is designed to operate in the wavelength range ofabout 1480 nm to about 1570 nm. The dopant is either Erbium or Erbiumco-doped with Ytterbium. For Erbium, typical values for the length 1603are in the range 5 to 100 meters, dopant concentration 1601 isapproximately 0.1% to 0.5% by weight, and the disposition 1602 is a ringaround the first core 1401 with an inner diameter of about one to twotimes the core diameter and a thickness of 2 to 5 microns. For Erbiumco-doped with Ytterbium, typical values for the length 1603 are in therange 0.5 to 50 meters, dopant concentration 1601 is approximatelyErbium 0.1% by weight and Ytterbium concentration is 10 to 20 times theErbium concentration, and the disposition 1602 is a ring around thefirst core 1401 with an inner diameter of about one to two times thecore diameter and a thickness of 2 to 5 microns. More examples are givenwith respect to FIGS. 6, 7 and 10. These design values are given forillustrative purposes only and are meant to be non-limiting. Forexample, it can be preferable in some instances to design a firstwaveguide structure 1401 with a very large first core 1402, in whichcase the length 1603 and the disposition 1602 would be different.

Another preferred embodiment is designed to operate in either thewavelength range of about 970 nm to 990 nm or the wavelength range ofabout 1010 nm to 1030 nm. The dopant is Ytterbium. Typical values forthe length 1603 are in the range 0.5 to 50 meters, dopant concentration1601 is approximately 0.1% to 2% by weight, and the disposition 1602 isa ring around the first core 1401 with an inner diameter of about one totwo times the core diameter and a thickness of 1 to 3 microns.

While the intrinsic fluorescence from Ytterbium in glass peaks around980 nm, it is well-known that cladding-pumped single-modedYtterbium-doped fiber lasers normally emit around 1060-1120 nm. Emissionat 980 nm is possible, but normally only at a significantly reducedefficiency. In contrast, a surprising and important result is that thedesign here allows efficient operation of cladding-pumped single-modedYtterbium-doped fiber lasers at 980 nm. The pump source 1450 ispreferably in the wavelength band from about 870 nm to about 950 nm, andpreferably between 900 and 940 nm. It is preferred that the amplifyingregion 1407 absorbs at least about 30% of the optical pump power 1406launched into the second waveguiding structure 1501.

Yet another preferred embodiment is designed to operate in thewavelength range of about 850 nm to 950 nm. The dopant is Neodymium.Typical values for the length 1603 are in the range 0.5 to 50 meters,dopant concentration 1601 is approximately 0.1% to 2% by weight, and thedisposition 1602 is a ring around the first core 1401 with an innerdiameter of about one to two times the core diameter and a thickness of1 to 3 microns.

FIG. 28 shows a cross-sectional view of a preferred embodiment of anoptical fiber 1610, having a ring-doped amplifying region 1407 locatedin the cladding 1403 and centered on the first core 1402. Also providedis a second core 1502 comprising the first core 1402 and cladding 1403.The second core 1502 is rectangularly shaped and is located within anouter cladding 2803. The refractive index of the first core 1402 ishigher than that of the cladding 1403, which is in turn higher than thatof the outer cladding 2803. The refractive index of the amplifyingregion can be equal to that of the cladding 1403 by way of co-dopants.The amplifying region 1407 can be doped with erbium, ytterbium, and/oranother rare earth element. The optical fiber 1610 can be made fromglass, preferably doped and undoped silica. The fiber can be surroundedby a coating made from a polymer or another material. Alternatively, theouter cladding 2803 can be made from a polymer.

It is preferred that the first core 1402 is single-moded at a desiredwavelength of optical radiation 1404.

FIG. 17 shows a schematic of a high-power optical amplifier 1700comprising at least one optical fiber 1610, an optical pump source 1405,a coupler 1701, an input port 1702, an output port 1703, a firstisolator 1704 and a second isolator 1705.

FIG. 17 also shows a filter 1706 that can be added in order to suppressamplified spontaneous emission at undesired wavelengths. The filter 1706can be a fiber optic Bragg grating, a long-period grating, an absorbingmedium, an acousto-optic filter, or an interferometric filter such asimplemented with a Mach Zehnder or a Fabry Perot. The filter 1706 can betunable and it can alternatively be placed following the output port1703.

The coupler 1701 can be a dichroic mirror, a wavelength divisionmultiplexing coupler, or a pump-injecting fused fiber coupler formed byside-coupling the core of a multimode optical fiber to the second core1502 by fusing and twisting the fibers together.

The optical pump source 1405 is preferably a high-power semiconductorlaser-diode coupled into the optical fiber 1610.

For high-power applications (greater than about 1 Watt), the secondisolator 1705 is preferably not utilized and the coupler 1701 ispreferably a dichroic mirror or a pump-injecting fused fiber coupler.Optical pump power 1406 can also be launched into the second waveguidingstructure 1501 through the side of the fiber, the pump power 1406 beingreflected into the second core 1502 by a V-groove formed in the fiber.Optical isolators are often used to suppress reflections in the signalbeam, and can also be used for protecting the optical pump source 1405.However, optical isolators are in general lossy and can also distort thebeam, and will therefore normally only be used if deemed necessary. Thisdepends on, for example, if high-gain strictly single-pass travelingwave amplification is required and also on if a pump source can bereached and damaged by cw or pulsed signal light, reflected pump light,or (in case of multiple pump sources) light from another pump source.

The different optical components in FIG. 17 and other embodiments can beoptical fiber devices or they can provide fiber pigtails for use asinput and output ports. If so, it is preferred that the differentcomponents are connected by splicing them together to form a continuousoptical fiber waveguide. Alternatively, it is possible to fabricateseveral optical components with a single optical fiber.

FIG. 18 shows a master oscillator power amplifier MOPA 1800 whichcomprises a master oscillator 1801 optically coupled to the firstwaveguiding structure 1401 of an optical amplifier 1802. The masteroscillator 1801 generates an optical seed 1803 which is amplified by theoptical amplifier 1802 to provide an output 1804 of higher power thanthe optical seed 1803.

The optical amplifier 1802 can be the optical amplifier 1700 shown inFIG. 17.

The master oscillator 1801 can be a semiconductor laser, a semiconductordistributed feedback laser, a fiber laser, a fiber distributed feedbacklaser, a fiber ring laser, a gas laser, a bulk laser, a source ofamplified spontaneous emission or a light emitting diode filtered by anoptical filter which can be an optical fiber Bragg grating.

The optical seed 1803 can be continuous wave, or can be an opticalpulsed seed.

In a preferred embodiment, the disposition of the amplifying region 1407is arranged such that the optical amplifier 1802 can store a largeamount of energy before it reaches its intrinsic lasing threshold. Bylarge, we mean large compared to a corresponding design in which theamplifying region 1407 is situated in the first core 1402. Opticalamplifiers designed in accordance with FIGS. 14 and 15 will performsurprisingly well for this application, far better than prior artoptical amplifiers having rare-earth dopants in the first core 1402.

The optical amplifier 1802 is preferably be the optical amplifier 1700which can be designed to have high energy storage at its intrinsiclasing threshold when used with an optical pulsed seed, the opticalamplifier 1700 being designed such that it is able to be operated suchthat the amplified optical pulse seed has an energy exceeding theintrinsic saturation energy of the optical amplifier 1700.

The optical amplifier 1802 preferably utilizes the optical fiberdescribed with FIG. 11, either in a core-pumped or a cladding-pumpedversion. The optical seed 1803 can also be provided by an externalsource not associated with the MOPA 1800. Thus the present inventionprovides for both the MOPA apparatus as well as the use of opticalamplifiers to amplify optical pulses to energies exceeding the intrinsicsaturation energy of the optical amplifier 1802.

The small-signal gain (without any optical radiation incident at theinput port) of the optical amplifier 1802 is normally no more than 40 dBand efficient energy extraction is difficult at a gain above 20 dB forpulsed operation. In order to reach the intrinsic saturation energy ofthe optical amplifier 1802, it is therefore preferred that the masteroscillator 1801 seeds the optical amplifier 1802 with pulses of at least0.01% and preferably at least 1% of the desired output pulse energy.Seeds 1803 with still higher pulse energy are preferable. Anintermediate amplifier can be required between the master oscillator1801 and the optical amplifier 1802 to ensure that the optical seed 1803is sufficiently large. The master oscillator 1801 can emit opticalpulses. Alternatively, there can be an optical time gate between themaster oscillator 1801 and the optical amplifier 1802 that opens andcloses to create an optical pulse pattern. It is preferred that the pumpsource 1405 is a semiconductor laser diode.

It is preferable to reduce the reflections outside the first waveguidingstructure 1401. This can be achieved by using antireflection coatings oran angled end to the first waveguiding structure 1401.

Similarly, for cw operation, the power of the optical seed 1803 thatseeds the optical amplifier 1802 should be at least 0.01% and preferablyat least 1% of the desired power of the output 1804. Still higher-powerseeds 1803 are preferable. An intermediate amplifier can be provided toreach adequate power levels for the optical seed 1803.

In a preferred embodiment, the optical amplifier 1802 comprises anoptical fiber 1610 containing an amplifying region 1407 that is dopedwith Ytterbium. The optical fiber 1610 is characterized by a dopantconcentration 1601, a disposition 1602 and a length 1603, and whereinthe dopant concentration 1601, the disposition 1602 and the length 1603of the amplifying region 1407 are configured such that the opticalamplifier 1802 amplifies in either the wavelength range of about 970 nmto 990 nm or the wavelength range of about 1010 nm to 1030 nm.

It is preferred that the amplifying region 1407 absorbs at least about30% of the optical pump power 1406 launched into the second waveguidingstructure 1501. Typical values for the length 1603 are in the range 0.5to 50 meters, dopant concentration 1601 is approximately 0.1% to 2% byweight, and the disposition 1602 is a ring around the first core 1401with an inner diameter of about one to two times the core diameter and athickness of 1 to 3 microns.

It is preferred that the bandwidth of the optical seed 1803 is greaterthan about 50 MHz in order to avoid complications arising from Brillouinscattering.

FIG. 19 shows a fiber laser 1900 comprising a pump source 1405, anamplifying optical waveguide structure 1408 and an optical feedbackdevice 1901, wherein the optical feedback device 1901 is configured toensure that a portion of the optical radiation 1404 guided by the firstwaveguiding structure 1401 is amplified more than once by any onesection of the amplifying region 1407.

The amplifying optical waveguide structure 1408 may be the claddingpumped amplifying optical waveguide structure 1508.

The optical feedback device 1901 may comprise a wavelength divisionmultiplexing coupler 1902, an isolator 1903 and an output coupler 1904.The optical pump power 1406 is launched into the first waveguidingstructure 1401 via the wavelength division multiplexing coupler 1902.This enables the optical radiation 1404 to circulate in a closed loopstructure 1910. The closed loop structure 1910 in this example is alaser cavity 1906 that is configured in a ring.

The wavelength division multiplexing coupler 1902 may be a dichroicmirror, or a fused wavelength division multiplexing fiber coupler

The fiber laser 1900 may be a core-pumped fiber laser with the opticalpump power 1406 being coupled into the first waveguiding structure 1401.

Higher output powers are obtainable with cladding pumping whereby theoptical pump power 1406 is coupled into the second waveguiding structure1501. Closed loop designs such as shown in FIG. 19 is preferably use adichroic mirror or a pump-injecting coupler formed by coupling the coreof a multimode optical fiber to the second core 1502 by fusing andtwisting the fibers together.

FIG. 20 shows a fiber laser 2000 where the optical feedback device 1901comprises at least one reflector 2005. The reflector 2005 is shown as afiber Bragg grating 2001 and a cleaved facet 2002. Alternatively,optical loop mirrors, dielectric mirrors, metallic mirrors or any otherform of reflector can be utilized. The two reflectors 2005 in FIG. 20and the amplifying optical waveguide structure 1408 are configured toform a laser cavity 1906, which in the example shown in FIG. 20 is alinear laser cavity. However, other types of laser cavity are alsopossible, such as ring cavities, A reflector 2005 can be partlytransmitting to form an output coupler 1904 through which a part of theoptical radiation 1404 that is incident on the reflector 2005 istransmitted, and emitted at an output port 1905 of the fiber laser 2000.In FIG. 20, the cleaved facet 2002 also forms the output coupler 1904.

The amplifying optical waveguide structure 1408 is preferably thecladding pumped amplifying optical waveguide structure 1508.

The reflector 2005 can be a diffraction grating placed externally to thefirst waveguiding structure 1401 and optically coupled to it.

The fiber Bragg grating 2001 is formed in the first waveguidingstructure 1401 so that it interacts with the optical radiation 1404guided by the first waveguiding structure 1401. The fiber Bragg grating2001 can be formed in the first core 1402.

It is preferable to reduce the reflections outside the desiredwavelength range. This can be achieved by using antireflection coatingsor an angled end to the first waveguiding structure 1401.

FIG. 21 shows a preferred embodiment of a fiber laser 2100. The fiberlaser 2100 comprises a laser cavity 1906. The optical fiber 1610comprises the amplifying optical waveguide structure 1408. Theamplifying region 1407 comprises Ytterbium and the fiber laser design istailored such that it emits in a wavelength region of about 970 nm toabout 990 nm. The design details of similar amplifier designs optimisedto amplify between about 970 nm to 990 nm were described in thedescription pertaining to FIG. 16.

Although the coupler 1701 is shown implemented as an optical fibercoupler, any of the implementations described previously can be used.The coupler 1701 is shown as being optically connected with a splice2101 to the optical fiber 1610, but this is not essential. It ispreferred to fabricate the coupler 1701 using the optical fiber 1610 forthe throughput connection.

It is preferred that the amplifying region 1407 is disposed in a ringsurrounding the first core 1402.

The optical radiation 1404 guided by the first waveguide structure 1401is characterized by an operating wavelength. It is preferred that thefirst waveguide structure 1401 is configured to support only onetransverse guided optical mode, albeit in two orthogonalpolarizations—note that in practice a single-mode optical fiber normallysupports two orthogonally polarized modes.

A lens 2102 can be used to collimate optical radiation emitted by theoptical fiber 1610 and coupled out from the laser cavity 1906. Theout-coupled optical radiation can be passed through an optical isolator2103.

The laser cavity 1906 can be configured in a linear configuration withthe optical feedback device 1901 comprising the fiber Bragg gratingreflector 2001 and the cleaved fiber facet 2002 shown in FIG. 21.

The optical feedback device 1901 can alternatively be provided by any ofthe implementations described previously, including a closed-loopstructure.

The cleaved fiber facet 2002 also constitutes an output coupler 1904through which optical radiation 404 is emitted from the laser cavity1906. Other options for an output coupler 1904 include any other partlytransmitting and partly reflecting arrangement like fiber Bragg gratingsand dielectric and metallic thin-film mirrors and also optical fiber andwaveguide couplers, which can split a beam traveling in a waveguide intotwo beams traveling in different waveguides. One beam can then remain inthe laser cavity 1906 while the other can be coupled out of the lasercavity 1906.

For improved wavelength selection and suppression of emission atunwanted wavelengths, it is preferred that the fiber Bragg grating 2001reflects predominantly at a desired wavelength of emission, while thereflectivity at undesired but amplified wavelengths is low, e.g., below0.1%. A lower value can be even better.

FIG. 22 shows a Q-switched laser 2200 comprising a pump source 1405, anamplifying optical waveguide structure 1408, at least one opticalfeedback device 1901, and an optical C switch 2201, wherein the opticalfeedback device 1901 is configured to ensure that a portion of theoptical radiation 1404 guided by the first waveguiding structure 1401 isamplified more than once by any one section of the amplifying region1407, the amplifying optical device being able to be operated such thatenergy is stored in the amplifying region 1407 with the optical switch2201 in a blocking state, the energy being released in the form of anoptical pulse when the optical switch 2201 is in a non-blocking state.

The optical switch 2201 is located inside the laser cavity 1906 in sucha way that it can block optical radiation 1404 propagating in the lasercavity 1906.

In FIG. 22, the optical feedback device 1901 are exemplified by a mirror2205 and a waveguide facet 2202 which also serves as a partly reflectingoutput coupler 1904. The waveguide facet 2202 is preferablyperpendicular.

Intracavity reflections can arise at any interface between two media.This is often undesired, in which case the reflections should besuppressed. For example, reflections from the amplifying opticalwaveguide structure can be suppressed by using the angled facet 2206illustrated in FIG. 22. Alternatively, a facet can be coated by ananti-reflection coating, e.g., in the form of a dielectric stack.

Reflections can also need to be suppressed in other types of amplifyingoptical devices 1400, and can be similarly suppressed.

The optical switch 2201 may not be physically connected to theamplifying optical waveguide structure 1408. In that case, anintracavity lens 2203 can be used to optically connect the opticalswitch 2201 to the amplifying optical waveguide structure 1408.

Optical pump power 1406 can be coupled from a pump source 1405 to theamplifying optical waveguide structure via a dichroic mirror 2204 and alens 2102. The dichroic mirror 2204 can also separate the optical pumppower 1406 and the optical radiation 1404 emitted from the amplifyingoptical waveguide structure 1408. The lens 2102 can also serve tocollimate the optical radiation 1404. Other means for coupling theoptical pump power 1406 into the amplifying optical waveguide 1408 canalternatively be used, including those previously discussed.

It is well known that Q-switched lasers can be operated in variousmodes, such as multiple pulses being emitted when the switch is opened,a single pulse being emitted, or a pulse being emitted on alternateswitch signals.

A preferred embodiment utilizes an optical fiber as the amplifyingoptical waveguide structure 1408.

Another preferred embodiment utilizes the optical fiber described withFIG. 11 as the amplifying optical waveguide structure 1408, either in acore-pumped or a cladding-pumped version.

The optical switch 2201 can be implemented using a waveguide switchfabricated from Lithium Niobate, Gallium Arsenide, or a fiber-opticacousto-optic switch, or a bulk optical switch such as an acousto opticmodulator, an acousto optic tunable filter, a Kerr cell, a Pockels cell,an elasto-optic modulator, or a liquid crystal switch.

The pump source 1405 can be a multi-moded semiconductor laser or ahigh-brightness, near-diffraction limited diode laser.

FIG. 23 shows a laser 2301 being used to pump at least one amplifyingoptical device 2300 via optical fibers 2302 and a power splitter 2303.The laser 2301 can be any laser designed in accordance with thisinvention.

The amplifying optical device 2300 is preferably be an Erbium DopedFiber Amplifier EDFA.

A preferred embodiment is the pumping of Erbium Doped Fiber AmplifiersEDFAs with the fibre laser 2100. It is preferred that the outputs frommore than one fiber laser 2100 are coupled together prior to the pumpingof the Erbium Doped Fiber Amplifiers in order to provide pumpredundancy.

Advantages of using the fibre laser 2100 for this application includehighly-efficient pumping of the Ytterbium ring-doped amplifying region1407 by relatively low-cost, high power and very-reliable multimodesemiconductor lasers for efficient generation of single-moded radiationin the wavelength band of about 970 nm to about 990 nm.

The amplifying optical devices 2300 can be conventional Erbium-dopedfiber amplifiers such as amplifiers installed in numerous numbers in thetelecommunication networks worldwide. Use of the fiber laser 2100 willallow the upgrading of such amplifiers to enable the amplification ofnumerous wavelength channels simultaneously at high output power (suchas required in dense wavelength division multiplexing systems).

The amplifying optical devices 2300 can also be Erbium Ytterbium dopedfiber amplifiers, or an amplifier designed in accordance with thisinvention, or any other optically pumped optical amplifiers includingplanar waveguiding ones.

The amplifying optical devices 2300 can also be optically pumped lasers,including fiber and planar distributed feedback lasers doped with erbiumand/or ytterbium configured for pumping by the fiber laser 2100.

FIG. 24 shows a waveguiding saturating absorber 2400 comprising awaveguiding structure 2401 having a core 2402 and a cladding 2403configured to guide optical radiation 2404 and an absorbing region 2405situated within the cladding 2403 and disposed such that it provides anabsorption of the optical radiation 2404 guided in the core 2402.

A preferred method to use the waveguiding saturating absorber 2400 iswhere in use at least 10% of the absorption is bleached by the opticalradiation 2404 guided by the waveguiding structure 2401 in at least apart of the waveguiding saturating absorber 2400 at least part of thetime

The waveguiding saturating absorber 2400 can absorb optical radiation2404 guided by the waveguiding structure 2401. However, at the sametime, power absorbed in the absorbing region 2405 bleaches theabsorption, which thus becomes smaller for optical radiation 2404 ofhigher power. The intrinsic saturation power P_(sat) is an importantparameter for a waveguiding saturating absorber. If the power of theoptical radiation 2404 is much smaller than P_(sat), the waveguidingsaturating absorber will be essentially unbleached and absorb a certainfraction of the optical radiation 2404. If the power of the opticalradiation 2404 is much larger than P_(sat) the waveguiding saturatingabsorber can be substantially bleached and unable to absorb any morepower, in which case the fraction of the power that is absorbeddecreases as the power increases.

By placing the absorbing region 2405 in the cladding 2403, the intrinsicsaturation power P_(sat) can be significantly higher, e.g., by one ortwo orders of magnitude, than if the absorbing region 2405 is placed inthe core 2402. This is often advantageous. Because of the tightconfinement of guided optical radiation 2404, (especially if thewaveguiding structure 2401 only supports a single transverse mode at thewavelength of the radiation 2404), the optical intensity inside the core2402 becomes high enough to bleach the absorbing region 2405 already atrelatively low optical powers. For instance, an erbium-doped fiber inwhich erbium doped in the core 2402 provides absorption normally has anintrinsic saturation power of less than 1 mW for optical radiation 2404around 1530 nm. It is often desirable to have a higher value of P_(sat)than that. A larger value of P_(sat) can be accomplished by placing theabsorbing region 2405 (e.g., doped with erbium) in the cladding 2403.Thereby, we can increase P_(sat) and furthermore by a careful choice ofthe location of the absorbing region 2405 achieve a specific value ofP_(sat) in order to optimize the characteristics of the waveguidingsaturating absorber 2400 in a particular application. In contrast, ifthe absorber (e.g., the erbium-doped medium) is spread evenly out in thecore of a standardized design, P_(sat) will primarily be determined bythe intrinsic properties of the absorber (e.g., the erbium dopedmedium), which are normally difficult to control.

Complex designs of the core 2402 can be used to increase the saturationpower P_(sat), (e.g. the effective size of the core 2402 can beincreased) the scope for change is much smaller than if the absorbingregion 2405 can reside in the cladding 2405. Complex designs of the coreinclude segmented cores, W-fibers, multiple cladding fibres, and coreswith so-called alpha profiles.

Any of the amplifying optical waveguide structures 1408 can, ifun-pumped, accordingly operate as a waveguiding saturating absorber 2400with a large and controllable intrinsic saturation power P_(sat) atwavelengths where the un-pumped gain region 1407 exhibits a saturableabsorption. The dopants used must therefore operate as a two or threelevel system at the operating wavelength of the waveguiding saturatingabsorber 2400. Specific examples of amplifying optical waveguidestructures are shown in FIGS. 14 and 15.

It is preferred that the saturable absorber is in a solid state.

It is preferred that the saturable absorber is a glass doped with a rareearth element.

A waveguiding saturating absorber 2400 can for instance be used forrejecting low-power radiation and as an optical switch in Q-switched andmode-locked lasers.

If the optical radiation 2404 is coherent and double-passed through thewaveguiding saturating absorber 2400 following reflection in one end ofthe waveguiding saturating absorber 2400, the optical radiation 2404 canform a standing-wave pattern which bleaches the waveguiding saturatingabsorber 2400 according to the standing-wave pattern. In this case,bleaching will predominantly occur at the wavelength of the opticalradiation 2404, while the absorption at neighboring wavelengths can behigher. This effect can be used in wavelength-tracking filters and forstabilization of single-frequency lasers.

FIG. 29 shows a wavelength-tracking filter 2900 having a waveguidingsaturating absorber 2400, a circulator 2901, and an optical feedbackdevice 1901, which can be a mirror 2205 butted directly to thewaveguiding saturating absorber 2400.

FIG. 30 shows a single-frequency laser 3000 comprising an amplifyingoptical structure 3008 and a waveguiding saturating absorber 2400. Theamplifying optical structure 3008 can be an amplifying opticalwaveguiding structure 1408, or a similar waveguiding structure but withthe amplifying region placed in the first core 1402. The amplifyingoptical structure 3008 can be pumped by a pump source 1405 which can belaunched via a wavelength division multiplexing coupler 1902. A lasercavity 1906 is formed by two optical feedback devices 1901. The opticalfeedback devices 1901 can be a fiber Bragg grating 2001 and a cleavedfacet 2002, or any other optical feedback device.

The amplifying optical structure 3008 and the waveguiding saturatingabsorber 2400 can be formed from separate structures joined together.Alternatively, if the amplifying optical structure 3008 is an amplifyingoptical waveguide structure 1408, they can be formed from a singleamplifying optical waveguide structure 1408, a part of which is pumpedand a part of which is unpumped. A part of the amplifying opticalwaveguide structure 1408 can remain unpumped because of a limited pumppenetration, or the configuration of the laser can be such that pumplight cannot reach the waveguiding saturating absorber 2400.

For a narrow-band wavelength-tracking filter, it is preferred that thewaveguiding saturating absorber 2400 is long, e.g., several centimetersup to several meters.

The waveguiding structure 2401 is preferably be single-moded and ispreferably a polarization maintaining or single-polarization waveguidingstructure.

FIG. 25a shows an amplifying optical device 1500 comprising a firstwaveguiding structure 1401 configured to guide optical radiation 1404which can propagate in a fundamental mode, a pump source 1405 configuredto supply optical pump power 1406, and a second waveguiding structure1501 configured to guide the optical pump power 1406. The pump source1405 is optically coupled to the second waveguiding structure 1501.

FIGS. 25b, 25 c and 25 d show optical power distributions 2510 of thefundamental mode of the optical radiation 1404 propagating alongdifferent first waveguide structures 1401.

In use, the optical radiation 1404 can be characterized by an opticalpower distribution 2510 of the fundamental mode having a contour 2505 ofequal intensity perpendicular to the local longitudinal axis of thefirst waveguiding structure 1401, the contour 2505 enclosing about 75%of the optical power of the fundamental mode. The area enclosed by thecontour 2505 defines a high intensity region 2501.

The second waveguiding structure 1501 contains an amplifying region 1407situated to interact with the optical pump power 1406 guided in thesecond waveguiding structure 1501 when the amplifying optical device1400 is in use; and wherein the amplifying region 1407 is situated tolie outside the contour 2505 of equal intensity.

It is preferred that during use at least 0.1% of the optical radiation1404 guided by the first waveguiding structure 1401 overlaps theamplifying region 1407.

In FIG. 25b, 25 c and 25 d, the contour 2505 and the high-intensityregion 2501 is exemplified with optical power distributions 2510 fromthree different kinds of first waveguiding structures 1401. FIG. 25bshows the first waveguiding structure 1401 of a normal circularsingle-moded step-index fiber. The high density region 2501 is circularand will large coincide with the core. The first core 1402 and possiblelocation of the amplifying region 1407 are also sketched in this case.FIG. 25c shows the optical power distribution 2510 which can bepropagated along a more complicated first waveguiding structure 1401.The high-density region 2501 is split up into main regions. FIG. 25dshows the optical power distribution 2510 which can be propagated alonga ring core fibre resulting in a ring-shaped high-intensity region 2501.

The first waveguiding structure 1401 can be a single mode waveguide, orcan support several (up to 10^(th) order) higher-order modes.

It is preferred that the disposition and design of the amplifying region1407 is arranged such that when in use, the intensity of the opticalradiation 1404 guided by the first waveguiding structure 1401 and theoptical pump power 1406 guided by the second waveguiding structure 1501are within the amplifying region 1407 on average approximately equal toother within about 10 dB. This averaging would be performed over thelongitudinal and transverse extent of the gain region 1407, and beweighted by the concentration of amplifying centers (e.g., Ytterbium orother rare-earth ions) in different parts of the amplifying region 1407.

Alternatively, it is preferred that the effective area ratior_(effective) is in the range 1 to 10.

It is preferred that at least 90% of the amplifying region 1407 islocated outside the high intensity region 2501. Insofar as theconcentration of the amplifying centers (e.g., Ytterbium or otherrare-earth ions) can vary within th amplifying region 1407, the meaningis understood to be that at least 90% of the amplifying centers shouldlie outside the high-intensity region 2501.

FIG. 26 shows a passive Q-switched laser 2600 comprising a pump source1405, a coupler 1701, a cladding-pumped amplifying optical waveguidestructure 1508 which is joined to an optical waveguide structure 2601doped with a saturable absorber 2603.

The design of the first waveguiding structure 1401 of thecladding-pumped amplifying optical waveguide structure 1508 can be thesame or similar to the design of the optical waveguide structure 2601.

The amplifying region 1407 in the cladding-pumped amplifying opticalwaveguide structure 1508 can be doped with the same active dopant as thesaturable absorber 2603 in the optical waveguide structure 2601. Theoperation of the passive Q-switched laser 2601 has been describedpreviously in the text.

The saturable absorber 2603 is preferably placed in the core of theoptical waveguide structure 2601.

The cladding-pumped amplifying optical waveguide structure 1508 isjoined (physically or optically) to the optical waveguide structure2601, so that the optical waveguide structure 2601 is optically coupledto the first waveguiding structure 1401 of the a cladding-pumpedamplifying optical waveguide structure 1508.

The cladding-pumped amplifying optical waveguide structure 1508 and theoptical waveguide structure 2601 can be optical fibers, and they can bejoined together by a splice 2101.

A laser cavity 1906 is formed with the cladding-pumped amplifyingoptical waveguide structure 1508 and the optical waveguide structure2601 and optical feedback device 1901.

The laser cavity 1906 can be of the linear type, as illustrated in FIG.26, in which case an output port 1703 and an output coupler 1904 can forexample be formed by the facet of the cladding-pumped amplifying opticalwaveguide structure 1508.

Alternatively, the optical fiber described with FIG. 11 can be used as apassively Q-switched fiber laser, in which case a saturable absorber ina first core 1402 and a amplifying medium 1407 doped in a cladding 1403or in the edges of the first core 1402 co-exist along the fiber, ratherthan being disposed in separate sections of fiber. Either core-pumpingor cladding-pumping can be use.

FIG. 27 shows a passive Q-switched laser 2700 comprising a pump source1405 end-coupled into an amplifying optical waveguide structure 1408which is joined via a pump-reflector 2702 to an optical waveguidestructure 2601 doped with a saturable absorber 2603.

The amplifying region 1407 in the amplifying optical waveguide structure1408 can be doped with the same active dopant as the saturable absorber2603 in the optical waveguide structure 2601. The design of the firstwaveguiding structure 1401 of the amplifying optical waveguide structure1508 can be the same or similar to the design of the optical waveguidestructure 2601. The saturable absorber 2603 is preferably placed in thecore of the optical waveguide structure 2601. The operation of thepassive Q-switched laser 2601 has been described previously in the text.

The pump reflector 2702 can be an optical fiber Bragg grating or anyother reflector which is transparent to the signal. Alternatively a pumpabsorber or a wavelength selective coupler can be used such as afused-fiber wavelength division multiplexing coupler in order toselectively transmit the signal in preference to the pump between thefirst waveguiding structure 1401 of the amplifying optical waveguidestructure 1408 and the optical waveguide structure 2601.

For pulsed laser emission, an optical feedback device 1901 and an outputcoupler 1904 must also be provided.

All of the features disclosed in this specification (including anyaccompanying claims, abstract, and drawings), and/or all of the steps ofany method or process so disclosed, can be combined in any combination,except combinations where at least some of such features are mutuallyexclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract, and drawings), can be replaced by alternative featuresserving the same, equivalent, or similar purpose, unless expresslystated otherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of the foregoingembodiments. The invention extends to any novel one, or any novelcombination, of the steps of any method or process so disclosed.

In the embodiments described above a ring-shaped (generally cylindrical)doped region has been used. However, the doped region does not of coursehave to be rotationally symmetric, nor evenly distributed along thelength of the fiber or waveguide.

While the above invention has been described with particularity tospecific embodiments and examples thereof, it is understood that theinvention comprises the general novel concepts disclosed by thedisclosure provided herein, as well as those specific embodiments andexamples, and should not be considered as limited by the specificembodiments and examples disclosed and described herein.

We claim:
 1. An amplifying optical device comprising: a first waveguidestructure comprising a first core and cladding and configured to guideoptical radiation; at least one pump source configured to supply opticalpump power; an amplifying region situated in the cladding; and whereinthe pump source is optically coupled to the amplifying region; andwherein in use the optical radiation guided in the first waveguidingstructure overlaps the amplifying region; and further comprising; asecond waveguiding structure comprising a second core and configured toguide the optical pump power; and wherein: the second waveguidingstructure contains the amplifying region; the second core is at leastpartly formed by at least part of the cladding; and the pump source isoptically coupled to the second waveguiding structure.
 2. An amplifyingoptical device according to claim 1 wherein the second core is adjacentto a region having a lower refractive index than the second core, theregion comprising at least one of a vacuum, a gas, a liquid, a polymerand a glass, the amplifying optical device being such that the regionprovides total internal reflection of the optical pump power.
 3. Anamplifying optical device according to claim 1 wherein the firstwaveguiding structure and the second waveguiding structure arefabricated in a single optical fiber.
 4. An amplifying optical deviceaccording to claim 1 wherein: the first waveguiding structure isfabricated from at least one glass system; the amplifying regioncontains a rare-earth dopant selected from the group consisting ofErbium and Erbium co-doped with Ytterbium; and the amplifying region ischaracterized by a dopant concentration, a disposition and a length, andwherein the dopant concentration, the disposition and the length of theamplifying region are arranged such that the amplifying optical deviceamplifies in the wavelength range of about 1480 nm to about 1570 nm. 5.An amplifying optical device according to claim 4 wherein the glasssystem is an oxide glass system selected from the group consisting ofsilica, doped silica, silicate, and phosphate.
 6. An amplifying opticaldevice according to claim 1 wherein: the first waveguiding structure isfabricated from at least one glass system; the amplifying region isdoped with Ytterbium; the pump source has a wavelength in the band fromabout 870 nm to about 950 nm; the amplifying region absorbs at leastabout 30% of the optical pump power launched into the second waveguidingstructure; and the amplifying region is characterized by a dopantconcentration of Ytterbium, a disposition and a length, and wherein thedopant concentration, the disposition and the length of the amplifyingregion are arranged such that the amplifying optical device amplifies ina wavelength range selected from the group of about 970 nm to about 990nm, and about 101 nm to 1030 nm.
 7. An amplifying optical deviceaccording to claim 1 and further comprising a master oscillatorconfigured to generate an optical seed, wherein the master oscillator isoptically coupled to the first waveguiding structure.
 8. An amplifyingoptical device comprising: a first waveguiding structure comprising afirst core and cladding and configured to guide optical radiation; atleast one pump source configured to supply optical pump power; anamplifying region situated in the cladding; and wherein the pump sourceis optically coupled to the amplifying region; and wherein in use theoptical radiation guided in the first waveguiding structure overlaps theamplifying region; and further comprising an optical feedback device,and wherein the optical feedback device is configured to ensure that aportion of the optical radiation guided by the first waveguidingstructure is amplified more than once by any one section of theamplifying region.
 9. An amplifying optical device according to claim 8wherein the optical feedback device comprises a closed-loop structure.10. An amplifying optical device according to claim 8 wherein theoptical feedback device comprises at least one reflector.
 11. Anamplifying optical device according to claim 6 further comprising anoptical feedback device, and wherein the optical feedback device isconfigured to ensure that a portion of the optical radiation guided bythe first waveguiding structure is amplified more than once by any onesection of the amplifying region.
 12. An amplifying optical deviceaccording to claim 11 wherein the optical radiation guided by the firstwaveguide structure is characterized by an operating wavelength, andwherein the amplifying region is disposed in a ring surrounding thefirst core, and wherein the first waveguide structure is configured tosupport only one transverse guided optical mode at its operatingwavelength.
 13. An amplifying optical device according to claim 8further comprising an optical switch, and wherein the amplifying opticaldevice is configured to be operable such that energy is stored in theamplifying region with the optical switch in a blocking state, theenergy being released in the form of at least one optical pulse when theoptical switch is in a non-blocking state.
 14. A method of pumping atleast one optical fiber amplifier with a fiber laser, the methodcomprising: providing a first waveguiding structure fabricated from atleast one glass system and comprising a first core and cladding;providing a second waveguiding structure comprising a second core atleast partly formed by the cladding and an amplifying region comprisingYtterbium; providing a source of optical pump power in opticalcommunication with the second waveguiding structure and having awavelength in the band from about 870 nm to about 950 nm; providing anoptical feedback device; guiding optical radiation using the firstwaveguiding structure; guiding the optical pump power using the secondwaveguiding structure such that the amplifying region interacts with theoptical radiation guided in the first waveguiding structure and theoptical pump power guided in the second waveguiding structure to amplifythe optical radiation guided by the first waveguiding structure; usingthe optical feedback device to ensure that a plurality of times aportion of the optical radiation guided by the first waveguidingstructure is amplified more than once by the amplifying region;providing an amplifying region characterized by a dopant concentration,a disposition and a length, and wherein the dopant concentration, thedisposition and the length of the amplifying region are arranged suchthat the fiber laser emits optical radiation at an emission wavelengthin the region of about 970 nm to about 990 nm; and coupling the opticalradiation at the emission wavelength in the region of about 970 nm to990 nm into the at least one optical amplifier.
 15. The method of claim14 wherein the amplifying region of the second waveguide ischaracterized in that it absorbs at least about 30% of the optical pumppower guided in the second waveguiding structure.
 16. A method ofamplifying optical pulses to energies exceeding the intrinsic saturationenergy of an amplifying optical device, comprising: providing a firstwaveguiding structure comprising a first core and cladding; providing asource of optical pump power; providing a second waveguiding structurecomprising a second core at least partly formed by at least part of thecladding, and an amplifying region; guiding optical radiation using thefirst waveguiding structure; and guiding the optical pump power usingthe second waveguiding structure such that the amplifying regioninteracts with the optical radiation guided in the first waveguidingstructure and the optical pump power guided in the second waveguidingstructure.
 17. An amplifying optical device comprising: a firstwaveguiding structure configured to guide optical radiation which canpropagate in a fundamental mode; a pump source configured to supplyoptical pump power; a second waveguiding structure configured to guidethe optical pump power, and wherein: the pump source is opticallycoupled to the second waveguiding structure; in use the opticalradiation is characterized by an optical power distribution of thefundamental mode having a contour of equal intensity perpendicular tothe local longitudinal axis of the first waveguiding structure thecontour enclosing about 75% of the optical power of the fundamentalmode; the second waveguiding structure contains an amplifying regionsituated to interact with the optical pump power guided in the secondwaveguiding structure when the amplifying optical device is in use; theamplifying region is situated to lie outside the contour of equalintensity; and during use at least 0.1% of the optical radiation guidedby the first waveguiding structure overlaps the amplifying region. 18.An amplifying optical device according to claim 17 wherein thedisposition and design of the amplifying region is arranged such thatwhen in use, the intensity of the optical radiation guided by the firstwaveguiding structure and the optical pump power guided by the secondwaveguiding structure are within the amplifying region on averageapproximately equal to each other within about 10 dB.
 19. An amplifyingoptical device according to claim 17 wherein at least 90% of theamplifying region is outside the high-intensity region through whichabout 75% of the power of the optical radiation guided by the firstwaveguiding structure propagates, and within which the intensity of theoptical radiation guided by the first core is everywhere higher than insurrounding regions.